Short answer: No.
Long answer: No because if you try, an angry African red-ass baboon will rape you until you scream the words "I am a newbie!" The client (you know, the thingie you double-click on before UO magically begins) only supports two map-sizes: the size of map0.mul and the size of map2.mul. Both sizes are fixed. The red-ass baboon likes bananas.
So if you really insist on having a smaller map, just fill most of it in with water or mountains or something. Avoid bananas and angry baboons. If you want a larger map, cover your ass and hope it doesn't take long.
EEEEEEEEEEEEEEEE!