change size to optional

src/ol/view.js

@@ -662,11 +662,13 @@ ol.View.prototype.getResolutions = function() {
 /**
  * Get the resolution for a provided extent (in map units) and size (in pixels).
  * @param {ol.Extent} extent Extent.
- * @param {ol.Size} size Box pixel size.
+ * @param {ol.Size} opt_size Box pixel size.
  * @return {number} The resolution at which the provided extent will render at
  *     the given size.
+ * @api
  */
-ol.View.prototype.getResolutionForExtent = function(extent, size) {
+ol.View.prototype.getResolutionForExtent = function(extent, opt_size) {
+  var size = opt_size || this.getSizeFromViewport_();
   var xResolution = ol.extent.getWidth(extent) / size[0];
   var yResolution = ol.extent.getHeight(extent) / size[1];
   return Math.max(xResolution, yResolution);