Fixed issue #82.

Author: killme2008

lib/query.js

@@ -387,6 +387,17 @@
       return this;
     },
 
+    /**
+     * Add a constraint to the query that requires a particular
+     * <strong>array</strong> key's length to be equal to the provided value.
+     * @param {String} key The array key to check.
+     * @param value The length value.
+     * @return {AV.Query} Returns the query, so you can chain this call.
+     */
+    sizeEqualTo: function(key, value) {
+      this._addCondition(key, "$size", value);
+    },
+
     /**
      * Add a constraint to the query that requires a particular key's value to
      * be not equal to the provided value.

test/query.js

@@ -141,6 +141,25 @@ describe("Queries",function(){
           throw error;
         }
       });
+
+
+    it("sizeEqualTo",function(done){
+      var gameScore = new GameScore();
+      gameScore.save({
+        players: ["a", "b"]
+      }).then(function() {
+        query = new AV.Query(GameScore);
+        query.sizeEqualTo("players", 2);
+        query.first({
+          success: function(object) {
+            expect(object.get('players').size).to.be(2);
+            done();
+          },
+          error: function(error) {
+            throw error;
+          }
+        });
+      });
     });
   });