Image Dimensions

So, I guess the heightmap's size is (2^a+1) x (2^b+1). And because you can set width and height, i would assume a can be != b.
But what about the lightmap and the coverage maps? Do they have to be heightmap's dimensions-1? Or, in other words, can I assume them to be (2^a) x (2^b), if the heightmap is (2^a+1) x (2^b+1)?

I do not want to implement unneccessary controls in my editor^^

Actually, they can be any size your graphics card can manage, since they are just textures the graphics card lays over the terrain vertices. For maximum compatibility you might want to choose square, power of two texture sizes. But the size you've given usually works alright and makes it easier to work with.