You can reduce the size of the models and thus make the map even bigger.
That itself would be a whole lot more trouble than it's worth, because it's not just reducing sizes. It's scaling down Doodads, reducing unit speeds, walk animations, the whole works. It's more easier to expand the map's size, which is unlikely.
BUT, if we DO get out of Blizzard, I can hope for additional cliff levels too.
/edit In that I mean cliff levels where you don't have to raise terrain and put custom walls and ramps, and where Reapers, Colossi, etc get to use their jump animations.