<script type="text/paperscript" canvas="quad">
  tool.maxDistance = 50;
  tool.minDistance = 4;

  function onMouseDrag(event) {
      var circle = new Path.Circle({
          center: event.middlePoint,
          radius: event.delta.length / 2
      });
      circle.fillColor = 'pink';
      circle.strokeColor = 'black';
  }
</script>
<canvas id="quad" resize></canvas>

* {
  padding: 0;
  margin: 0;
}

canvas {
  width: 850px;
  height: 400px;
  background: lightgray;
}