It is not possible to change the size of map0.mul without editing the client. You can, however, make a smaller map within map0.mul and simply disallow access to most of it using mountain ranges, server settings, and the like.
Some people have attempted to ?crack? into the client and change the hard-coded size of map0.mul, see this thread:
http://www.ryandor.com/forum/viewtopic. ... sc&start=0
In addition, there are some unofficial clients (not made by EA) that support any size of map0.mul. I believe this project will support any size (but I could be wrong, and it looks inactive):
http://www.darkcloaks.co.uk/Aurora/