I've only really just been dabbling with WM until I started on the same thing. I got all keen and splashed out on the professional version to try and build my own reimagining of Britannia based on the UO map. Eventually I plan on importing into the whole thing into Unreal Engine.
I've had to make a couple of compromises with size. the whole map (including all the islands) works out at 96km x 96km and working on the basis of importing 2km tiles. While it's not going to be my view of a life-sized Britannia, when you consider that where Empath Abbey would be to the nearest mountains is like 8-10km, it's one hell of a hike for a player ... and that's a lot of forest.
And I've had to invent a few landmarks to make the maps a bit more interesting as well. Even with my compromises, there's a lot of space to fill and the UO map is a little sparse in interesting detail.
Below is a link to a couple of screenshots so far. I've been concentrating on the Yew area, but sculpted Buccaneer's Den early on to get some ideas and techniques going. I had to restart 3 times before I got it right.