title: changing the icon size with map zooming
body: <p>I'm using a icon 20x20 pxl to be displayed at the map with below code:</p>
  
  <p>When I zoom the map, the icon appears to be small, how can I change the icon size with map zooming?</p>
  
  <pre class="lang-html prettyprint-override"><code> &lt;link rel="stylesheet" href="https://openlayers.org/en/v4.6.5/css/ol.css" type="text/css"&gt;
   &lt;script src="https://openlayers.org/en/v4.6.5/build/ol.js" type="text/javascript"&gt;&lt;/script&gt;
  
  &lt;script&gt;
  var baseMapLayer = new ol.layer.Tile({
      source: new ol.source.OSM()
    });
  
  var view = new ol.View({
              center: ol.proj.fromLonLat([-74.0061,40.712]), 
              zoom: 17 //Initial Zoom Level
  })
  
  var map = new ol.Map({
      target: 'map',
      layers: [ baseMapLayer],
      view: view
  });
  
  // Make a new feature marker
  var marker = new ol.Feature({
      name: 'Null Island',
      population: 4000,
      rainfall: 500,
      geometry: new ol.geom.Point(
        ol.proj.fromLonLat([-74.006,40.7127])  // new Point([0, 0])
      ),  // Cordinates of New York's Town Hall
  });
  
  // Style the marker
  var iconStyle = new ol.style.Style({
      image: new ol.style.Icon(({
          color: '#ffcd46',
          crossOrigin: 'anonymous',
          src: 'http://127.0.0.1:8081/static/img/truck.png'
      }))
  });
  
  marker.setStyle(iconStyle);
  
  // Make a source for the feature 
  var vectorSource = new ol.source.Vector({
      features: [marker]
  });
  // Make a new layer
  var markerVectorLayer = new ol.layer.Vector({
      source: vectorSource,
  });
  
  map.addLayer(markerVectorLayer);
  &lt;/script&gt;
  </code></pre>