d3 select and grow node in force directed graph.

<!DOCTYPE html>
<html>

  <head>
    <meta charset="utf-8" />
    <meta name="viewport" content="initial-scale=1,maximum-scale=1" />
    <title>Force Directed Graph</title>
    <script data-require="d3@*" data-semver="3.4.6" src="//cdnjs.cloudflare.com/ajax/libs/d3/3.4.6/d3.min.js"></script>
    <link rel="stylesheet" type="text/css" href="styles.css" />
  </head>

  <body>
  <p>Enter a number between 0 and 250</p>
    <input id="targetNode" name="targetNode" type="text" />
    <button onclick="growNode()">grow node</button>
    <button onclick="shrinkNode()">shrink node</button>
    <script type="text/javascript">

    var w = 600,
        h = 600,
        z = d3.scale.category20();

    var force = d3.layout.force()
        .size([w, h]);

    var svg = d3.select("body").append("svg")
        .attr("width", w)
        .attr("height", h);

    d3.json("flare.json", function(root) {
      var nodes = flatten(root),
          links = d3.layout.tree().links(nodes);

      force
          .nodes(nodes)
          .links(links)
          .start();

      var link = svg.selectAll("line")
          .data(links)
        .enter().insert("line")
          .style("stroke", "#999")
          .style("stroke-width", "1px");

      var enterNode = svg.selectAll("circle")
          .data(nodes)
        .enter().append("circle")
          .attr("id", function(d, i){
              //nodes[i].id = "c" + i;
              return "c" + i;});

        // UPDATE
      var node =  svg.selectAll("circle")
          .attr("r", 4.5)
          .style("fill", function(d) { return z(d.parent && d.parent.name); })
          .style("stroke", "#000")
          .call(force.drag);

      force.on("tick", function(e) {
        link.attr("x1", function(d) { return d.source.x; })
            .attr("y1", function(d) { return d.source.y; })
            .attr("x2", function(d) { return d.target.x; })
            .attr("y2", function(d) { return d.target.y; });

        node.attr("cx", function(d) { return d.x; })
            .attr("cy", function(d) { return d.y; });
      });
    });

    function flatten(root) { // <-A
      var nodes = [];
      function traverse(node, depth) {
        if (node.children) {
          node.children.forEach(function(child) {
            child.parent = node;
            traverse(child, depth + 1);
          });
        }
        node.depth = depth;
        nodes.push(node);
      }
      traverse(root, 1);
      return nodes;
    }

    function growNode(){
        var userInput = document.getElementById("targetNode");
        var theNode = d3.select("#c"+userInput.value);
        theNode.attr("r", 20);
    }
    function shrinkNode(){
        var userInput = document.getElementById("targetNode");
        var theNode = d3.select("#c"+userInput.value);
        theNode.attr("r", 4.5);
    }
    </script>
  </body>

</html>

// Code goes here

/* Styles go here */

body {
    font-family: "helvetica";
}

button {
    margin: 0 7px 0 0;
    background-color: #f5f5f5;
    border: 1px solid #dedede;
    border-top: 1px solid #eee;
    border-left: 1px solid #eee;

    font-size: 12px;
    line-height: 130%;
    text-decoration: none;
    font-weight: bold;
    color: #565656;
    cursor: pointer;
}

.box {
    width: 200px;
    height: 200px;
    margin: 40px;
    float: left;
    text-align: center;
    border: #969696 solid thin;
    padding: 5px;
}

.red {
    background-color: #e9967a;
    color: #f0f8ff;
}

.blue {
    background-color: #add8e6;
    color: #f0f8ff;
}

.cell {
    min-width: 40px;
    min-height: 20px;
    margin: 5px;
    float: left;
    text-align: center;
    border: #969696 solid thin;
    padding: 5px;
}

.fixed-cell {
    min-width: 40px;
    min-height: 20px;
    margin: 5px;
    position: fixed;
    text-align: center;
    border: #969696 solid thin;
    padding: 5px;
}

.h-bar {
    min-height: 15px;
    min-width: 10px;
    background-color: steelblue;
    margin-bottom: 2px;
    font-size: 11px;
    color: #f0f8ff;
    text-align: right;
    padding-right: 2px;
}

.v-bar {
    min-height: 1px;
    min-width: 30px;
    background-color: #4682b4;
    margin-right: 2px;
    font-size: 10px;
    color: #f0f8ff;
    text-align: center;
    width: 10px;
    display: inline-block;
}

.baseline {
    height: 1px;
    background-color: black;
}

.clear {
    clear: both;
}

.selected {
    background-color: #f08080;
}

.control-group {
    padding-top: 10px;
    margin: 10px;
}

.table {
    width: 70%;
}

.table td, th {
    padding: 5px;
}

.table-header {
    background-color: #00AFEF;
    font-weight: bold;
}

.table-row-odd {
    background-color: #f0f8ff;
}

.table-row-odd {
    background-color: #d3d3d3;
}

.code {
    display: inline-block;
    font-style: italic;
    background-color: #d3d3d3;
    border: #969696 solid thin;
    padding: 10px;
    margin-top: 10px;
    margin-bottom: 10px;
}

.countdown{
    width: 150px;
    height: 150px;
    font-size: 5em;
    font-weight: bold;
}

.axis path, .axis line {
    fill: none;
    stroke: #000;
    shape-rendering: crispEdges;
}

.axis text {
    font: 10px sans-serif;
}

.axis .grid-line{
    stroke: black;
    shape-rendering: crispEdges;
    stroke-opacity: .2;
}

.line{
    fill: none;    
    stroke: steelblue;
    stroke-width: 2;
}

.dot {
  fill: #fff;
  stroke: steelblue;
}

.area {
    stroke: none;
    fill: steelblue;
    fill-opacity: .2;
}

.pie text{
    fill: white;
    font-weight: bold;
}

.circle {
    stroke: none;
    fill: red;
    fill-opacity: .7;
}

.cross {
    stroke: none;
    fill: blue;
    fill-opacity: .7;
}

.diamond {
    stroke: none;
    fill: green;
    fill-opacity: .7;
}

.square{
    stroke: none;
    fill: yellow;
    fill-opacity: .7;
}

.triangle-down{
    stroke: none;
    fill: blueviolet;
    fill-opacity: .7;
}

.triangle-up{
    stroke: none;
    fill: darkred;
    fill-opacity: .7;
}

.bubble{
    fill-opacity: .3;
}

.bar{
    stroke: none;
    fill: steelblue;
}


{
 "name": "flare",
 "children": [
  {
   "name": "analytics",
   "children": [
    {
     "name": "cluster",
     "children": [
      {"name": "AgglomerativeCluster", "size": 3938},
      {"name": "CommunityStructure", "size": 3812},
      {"name": "HierarchicalCluster", "size": 6714},
      {"name": "MergeEdge", "size": 743}
     ]
    },
    {
     "name": "graph",
     "children": [
      {"name": "BetweennessCentrality", "size": 3534},
      {"name": "LinkDistance", "size": 5731},
      {"name": "MaxFlowMinCut", "size": 7840},
      {"name": "ShortestPaths", "size": 5914},
      {"name": "SpanningTree", "size": 3416}
     ]
    },
    {
     "name": "optimization",
     "children": [
      {"name": "AspectRatioBanker", "size": 7074}
     ]
    }
   ]
  },
  {
   "name": "animate",
   "children": [
    {"name": "Easing", "size": 17010},
    {"name": "FunctionSequence", "size": 5842},
    {
     "name": "interpolate",
     "children": [
      {"name": "ArrayInterpolator", "size": 1983},
      {"name": "ColorInterpolator", "size": 2047},
      {"name": "DateInterpolator", "size": 1375},
      {"name": "Interpolator", "size": 8746},
      {"name": "MatrixInterpolator", "size": 2202},
      {"name": "NumberInterpolator", "size": 1382},
      {"name": "ObjectInterpolator", "size": 1629},
      {"name": "PointInterpolator", "size": 1675},
      {"name": "RectangleInterpolator", "size": 2042}
     ]
    },
    {"name": "ISchedulable", "size": 1041},
    {"name": "Parallel", "size": 5176},
    {"name": "Pause", "size": 449},
    {"name": "Scheduler", "size": 5593},
    {"name": "Sequence", "size": 5534},
    {"name": "Transition", "size": 9201},
    {"name": "Transitioner", "size": 19975},
    {"name": "TransitionEvent", "size": 1116},
    {"name": "Tween", "size": 6006}
   ]
  },
  {
   "name": "data",
   "children": [
    {
     "name": "converters",
     "children": [
      {"name": "Converters", "size": 721},
      {"name": "DelimitedTextConverter", "size": 4294},
      {"name": "GraphMLConverter", "size": 9800},
      {"name": "IDataConverter", "size": 1314},
      {"name": "JSONConverter", "size": 2220}
     ]
    },
    {"name": "DataField", "size": 1759},
    {"name": "DataSchema", "size": 2165},
    {"name": "DataSet", "size": 586},
    {"name": "DataSource", "size": 3331},
    {"name": "DataTable", "size": 772},
    {"name": "DataUtil", "size": 3322}
   ]
  },
  {
   "name": "display",
   "children": [
    {"name": "DirtySprite", "size": 8833},
    {"name": "LineSprite", "size": 1732},
    {"name": "RectSprite", "size": 3623},
    {"name": "TextSprite", "size": 10066}
   ]
  },
  {
   "name": "flex",
   "children": [
    {"name": "FlareVis", "size": 4116}
   ]
  },
  {
   "name": "physics",
   "children": [
    {"name": "DragForce", "size": 1082},
    {"name": "GravityForce", "size": 1336},
    {"name": "IForce", "size": 319},
    {"name": "NBodyForce", "size": 10498},
    {"name": "Particle", "size": 2822},
    {"name": "Simulation", "size": 9983},
    {"name": "Spring", "size": 2213},
    {"name": "SpringForce", "size": 1681}
   ]
  },
  {
   "name": "query",
   "children": [
    {"name": "AggregateExpression", "size": 1616},
    {"name": "And", "size": 1027},
    {"name": "Arithmetic", "size": 3891},
    {"name": "Average", "size": 891},
    {"name": "BinaryExpression", "size": 2893},
    {"name": "Comparison", "size": 5103},
    {"name": "CompositeExpression", "size": 3677},
    {"name": "Count", "size": 781},
    {"name": "DateUtil", "size": 4141},
    {"name": "Distinct", "size": 933},
    {"name": "Expression", "size": 5130},
    {"name": "ExpressionIterator", "size": 3617},
    {"name": "Fn", "size": 3240},
    {"name": "If", "size": 2732},
    {"name": "IsA", "size": 2039},
    {"name": "Literal", "size": 1214},
    {"name": "Match", "size": 3748},
    {"name": "Maximum", "size": 843},
    {
     "name": "methods",
     "children": [
      {"name": "add", "size": 593},
      {"name": "and", "size": 330},
      {"name": "average", "size": 287},
      {"name": "count", "size": 277},
      {"name": "distinct", "size": 292},
      {"name": "div", "size": 595},
      {"name": "eq", "size": 594},
      {"name": "fn", "size": 460},
      {"name": "gt", "size": 603},
      {"name": "gte", "size": 625},
      {"name": "iff", "size": 748},
      {"name": "isa", "size": 461},
      {"name": "lt", "size": 597},
      {"name": "lte", "size": 619},
      {"name": "max", "size": 283},
      {"name": "min", "size": 283},
      {"name": "mod", "size": 591},
      {"name": "mul", "size": 603},
      {"name": "neq", "size": 599},
      {"name": "not", "size": 386},
      {"name": "or", "size": 323},
      {"name": "orderby", "size": 307},
      {"name": "range", "size": 772},
      {"name": "select", "size": 296},
      {"name": "stddev", "size": 363},
      {"name": "sub", "size": 600},
      {"name": "sum", "size": 280},
      {"name": "update", "size": 307},
      {"name": "variance", "size": 335},
      {"name": "where", "size": 299},
      {"name": "xor", "size": 354},
      {"name": "_", "size": 264}
     ]
    },
    {"name": "Minimum", "size": 843},
    {"name": "Not", "size": 1554},
    {"name": "Or", "size": 970},
    {"name": "Query", "size": 13896},
    {"name": "Range", "size": 1594},
    {"name": "StringUtil", "size": 4130},
    {"name": "Sum", "size": 791},
    {"name": "Variable", "size": 1124},
    {"name": "Variance", "size": 1876},
    {"name": "Xor", "size": 1101}
   ]
  },
  {
   "name": "scale",
   "children": [
    {"name": "IScaleMap", "size": 2105},
    {"name": "LinearScale", "size": 1316},
    {"name": "LogScale", "size": 3151},
    {"name": "OrdinalScale", "size": 3770},
    {"name": "QuantileScale", "size": 2435},
    {"name": "QuantitativeScale", "size": 4839},
    {"name": "RootScale", "size": 1756},
    {"name": "Scale", "size": 4268},
    {"name": "ScaleType", "size": 1821},
    {"name": "TimeScale", "size": 5833}
   ]
  },
  {
   "name": "util",
   "children": [
    {"name": "Arrays", "size": 8258},
    {"name": "Colors", "size": 10001},
    {"name": "Dates", "size": 8217},
    {"name": "Displays", "size": 12555},
    {"name": "Filter", "size": 2324},
    {"name": "Geometry", "size": 10993},
    {
     "name": "heap",
     "children": [
      {"name": "FibonacciHeap", "size": 9354},
      {"name": "HeapNode", "size": 1233}
     ]
    },
    {"name": "IEvaluable", "size": 335},
    {"name": "IPredicate", "size": 383},
    {"name": "IValueProxy", "size": 874},
    {
     "name": "math",
     "children": [
      {"name": "DenseMatrix", "size": 3165},
      {"name": "IMatrix", "size": 2815},
      {"name": "SparseMatrix", "size": 3366}
     ]
    },
    {"name": "Maths", "size": 17705},
    {"name": "Orientation", "size": 1486},
    {
     "name": "palette",
     "children": [
      {"name": "ColorPalette", "size": 6367},
      {"name": "Palette", "size": 1229},
      {"name": "ShapePalette", "size": 2059},
      {"name": "SizePalette", "size": 2291}
     ]
    },
    {"name": "Property", "size": 5559},
    {"name": "Shapes", "size": 19118},
    {"name": "Sort", "size": 6887},
    {"name": "Stats", "size": 6557},
    {"name": "Strings", "size": 22026}
   ]
  },
  {
   "name": "vis",
   "children": [
    {
     "name": "axis",
     "children": [
      {"name": "Axes", "size": 1302},
      {"name": "Axis", "size": 24593},
      {"name": "AxisGridLine", "size": 652},
      {"name": "AxisLabel", "size": 636},
      {"name": "CartesianAxes", "size": 6703}
     ]
    },
    {
     "name": "controls",
     "children": [
      {"name": "AnchorControl", "size": 2138},
      {"name": "ClickControl", "size": 3824},
      {"name": "Control", "size": 1353},
      {"name": "ControlList", "size": 4665},
      {"name": "DragControl", "size": 2649},
      {"name": "ExpandControl", "size": 2832},
      {"name": "HoverControl", "size": 4896},
      {"name": "IControl", "size": 763},
      {"name": "PanZoomControl", "size": 5222},
      {"name": "SelectionControl", "size": 7862},
      {"name": "TooltipControl", "size": 8435}
     ]
    },
    {
     "name": "data",
     "children": [
      {"name": "Data", "size": 20544},
      {"name": "DataList", "size": 19788},
      {"name": "DataSprite", "size": 10349},
      {"name": "EdgeSprite", "size": 3301},
      {"name": "NodeSprite", "size": 19382},
      {
       "name": "render",
       "children": [
        {"name": "ArrowType", "size": 698},
        {"name": "EdgeRenderer", "size": 5569},
        {"name": "IRenderer", "size": 353},
        {"name": "ShapeRenderer", "size": 2247}
       ]
      },
      {"name": "ScaleBinding", "size": 11275},
      {"name": "Tree", "size": 7147},
      {"name": "TreeBuilder", "size": 9930}
     ]
    },
    {
     "name": "events",
     "children": [
      {"name": "DataEvent", "size": 2313},
      {"name": "SelectionEvent", "size": 1880},
      {"name": "TooltipEvent", "size": 1701},
      {"name": "VisualizationEvent", "size": 1117}
     ]
    },
    {
     "name": "legend",
     "children": [
      {"name": "Legend", "size": 20859},
      {"name": "LegendItem", "size": 4614},
      {"name": "LegendRange", "size": 10530}
     ]
    },
    {
     "name": "operator",
     "children": [
      {
       "name": "distortion",
       "children": [
        {"name": "BifocalDistortion", "size": 4461},
        {"name": "Distortion", "size": 6314},
        {"name": "FisheyeDistortion", "size": 3444}
       ]
      },
      {
       "name": "encoder",
       "children": [
        {"name": "ColorEncoder", "size": 3179},
        {"name": "Encoder", "size": 4060},
        {"name": "PropertyEncoder", "size": 4138},
        {"name": "ShapeEncoder", "size": 1690},
        {"name": "SizeEncoder", "size": 1830}
       ]
      },
      {
       "name": "filter",
       "children": [
        {"name": "FisheyeTreeFilter", "size": 5219},
        {"name": "GraphDistanceFilter", "size": 3165},
        {"name": "VisibilityFilter", "size": 3509}
       ]
      },
      {"name": "IOperator", "size": 1286},
      {
       "name": "label",
       "children": [
        {"name": "Labeler", "size": 9956},
        {"name": "RadialLabeler", "size": 3899},
        {"name": "StackedAreaLabeler", "size": 3202}
       ]
      },
      {
       "name": "layout",
       "children": [
        {"name": "AxisLayout", "size": 6725},
        {"name": "BundledEdgeRouter", "size": 3727},
        {"name": "CircleLayout", "size": 9317},
        {"name": "CirclePackingLayout", "size": 12003},
        {"name": "DendrogramLayout", "size": 4853},
        {"name": "ForceDirectedLayout", "size": 8411},
        {"name": "IcicleTreeLayout", "size": 4864},
        {"name": "IndentedTreeLayout", "size": 3174},
        {"name": "Layout", "size": 7881},
        {"name": "NodeLinkTreeLayout", "size": 12870},
        {"name": "PieLayout", "size": 2728},
        {"name": "RadialTreeLayout", "size": 12348},
        {"name": "RandomLayout", "size": 870},
        {"name": "StackedAreaLayout", "size": 9121},
        {"name": "TreeMapLayout", "size": 9191}
       ]
      },
      {"name": "Operator", "size": 2490},
      {"name": "OperatorList", "size": 5248},
      {"name": "OperatorSequence", "size": 4190},
      {"name": "OperatorSwitch", "size": 2581},
      {"name": "SortOperator", "size": 2023}
     ]
    },
    {"name": "Visualization", "size": 16540}
   ]
  }
 ]
}