Well, if you are simply editing the map with the World Editor then yes (use lightning brigade's instructions for that).
If you are talking about MAP INITIALIZATION then that's different, and I do not think that you can change the size, but you can add boundaries with a trigger. So if you want to reduce or add to the map size, simply make the map as big as it can be, and then take away map boundaries with a trigger to make it "bigger" during gameplay.