create an array of unknown size in a loop

Hi there, I have a loop that fills an array. basically I go through a simple puzzle game level, look for vacant locations and write the location in the list. Later, I pick a random location from the list and do something.

 var availableLocations=new Block[xwidth*ywidth];
 
         var internalLoop:int=0;
         //x axis loop
         for(x=1;x<ywidth-1;x++)
         {
             //y axis loop
             for(y=1;y<ywidth-1;y++)
             {
                 if(levelArray[x,y].type==BlockType.Nothing) //check if there is nothing yet at the location
                 {
                     availableLocations[internalLoop]=levelArray[x,y]; //add the location to the list of available locations
                     internalLoop+=1;
                 }
             }
         }
 
 //create a goal block at one of the available locations
         var r=Mathf.Floor(Random.value*availableLocations.length);
         print("trying to create a gloal block at location"+r);
         var goalBlock=availableLocations[r];
         goalBlock.type=BlockType.Goal;
         goalBlock.color=levelColors[currentGoalBlockColor];

the problem is, that my array is too one and I sometimes get nullReferenceExceptions because I tried to access an object that wasn't written in my array because the array is as big as the whole level, but the amount of vacant locations is only a part of the level.

note: at the last line I use a method of the Block at the picked location in the array. I was thinking about using the native javascript array, but the problem is that, if I do something like Array[].method with it, I get an error because the items are generic objects instead of the classed i've used so I can't use methods (at least as far as I now I can't as I tried)

How can I make availableLocations[] be only as big as internalLoop gets during the loop?

thanks in advance!