Tired of being restricted to 7168x4096 tiles to construct your custom UO world on? Well be tired no more, because Bmzx007 has come up with a way to use custom sized maps in the client, which theoretically supports mapsizes up to 32000x32000, for
each map#.mul file.
This currently requires manually using a hex-editor to change some values in client.exe, but utilities that help you with both that and expanding the actual size of the map-files are in the works.
You can view the complete details here:
HowTo: Custom Map Sizes & Map Wrap Points