iconCreateFunction: function (cluster) {
 var childCount = cluster.getChildCount();
 var c = ' marker-cluster-';
 if (childCount < 10) {
   c += 'small';
 } 
 else if (childCount < 100) {
   c += 'medium';
 } 
 else {
   c += 'large';
 }

 return new L.DivIcon({ html: '
' + childCount + '
', 
  className: 'marker-cluster' + c, iconSize: new L.Point(40, 40) });
 }