I'm not sure, some people does say that their map maker only can read 513 x 513
But you should be able to decrease the size or increase it as long as it's the same on both sides. Example = 1024 x 1024
I don't know if you already know this, but the size of the picture doesn't really make your map bigger. The size is changed from within the map maker.
If you do increase the image size you might be able to get more precise hills and slopes if you can work it well.