title: OpenLayers 3 Resize a feature
body: <p>I am using OpenLayers 3 and OpenStreetMap for my application.
  I've managed to draw circle vectors on the map to draw markers, the problem I have is that the circles have always the same size when zooming in and out.
  How can I change the size of the vectors according to the resolution ?</p>
  
  <p>Here is my vector definition:</p>
  
  <pre><code>var dealerSource = new ol.source.Vector();
  function dealerStyle(feature) {
      var style = new ol.style.Style({
          image: new ol.style.Circle({
              radius: 6,
              stroke: new ol.style.Stroke({
                  color: 'white',
                  width: 2
              }),
              anchor: [1.5, 1.5],
              fill: new ol.style.Fill({
                  color: 'green'
              })
          })
      });
      return [style];
  }
  </code></pre>