Radius

Node size is controlled by the radius attribtue.
          <!DOCTYPE html>
<html>

<head>
  <meta charset="utf-8">
  <script src="../build/ogma.min.js"></script>
  <style>
    #graph-container {
      top: 0;
      bottom: 0;
      left: 0;
      right: 0;
      position: absolute;
      margin: 0;
      overflow: hidden;
    }
  </style>
</head>

<body>
  <div id="graph-container"></div>

  <script>
    'use strict';

    var ogma = new Ogma({
      container: 'graph-container'
    });

    // Specify a random size for each node.
    ogma.styles.addNodeRule({
      radius: function (n) { return 3 + (n.getId() % 10); }
    });

    // Generate a random graph.
    ogma.generate.random({ nodes: 50, edges: 0 })
      .then(function (graph) {

        // Assign the graph to Ogma.
        ogma.setGraph(graph);
        ogma.view.locateGraph();

        // Change the size of a single node and its color to purple.
        ogma.getNode('0')
          .setAttributes({ radius: 5, color: 'purple' });

        // Change the size of two nodes and their color to orange.
        ogma.getNodes(['1', '2'])
          .setAttributes({ radius: 8, color: 'orange' });
      });
  </script>
</body>

</html>