you can use any size of map...within certain restictions;
- Map dimensions are resized downwards to powers of 2 IIRC - eg an 800x800 map will be downsized to 512x512 by the 3d card. I think it's partly to do with memory allocation. Also, modelview uses openGL, so any maps NOT in powers of 2 won't display properly - it'll look like you're mapping is crewed. Seeing as the 3d cards aren't great at this downsizing, and probably still store the original texture themselves, it's best to resize - upwards or downwards - yourself.
Important bit!!!!
- Without the registry edits, maps are always downsized to 256x256 if they are bigger. So you can use the big maps, but they'll lose res.
End of Important bit!!!!
Because the 3d card isn't as good at resizing as a paint prog, it's best you provide hi and lo res version of the maps. The other thing is that big maps will take up the same space regardless of whether they are resized or not, which could lead to slowdown with older cards (use of texture RAM).