global way of measuring size of game objects

I'm trying to put a primitive unity cube exactly above the shelf shown in the image (it was taken out of its model) so the equation iam using is box's position = (shelf's position.x , shelf's position.y + (shelf size in y axes /2 ) + ( box's size in y axes /2 ) , shelf's postion.z) but neither lossyscale nor local scale return the desired size. so as shown in the image i want to measure the size of the object relative to the global 1*1 square grid so for the z axes it should return 2 and for the z axes it should return .75 . ![alt text][1] [1]: /storage/temp/88011-untitled.png