For Wilbur, do you know what a plausible maximum size in pixels is? It crashes if I try to do 1.5 million pixels across -- I know I don't necessarily need 1m scale for the high level map, but the more detailed I can make it the better. Also, how do I tell Wilbur how many metres each pixel represents? I assume that is necessary knowledge for e.g. erosion algorithms to work?

I'm trying to decide whether to just bite the bullet and accept I'll be working on smaller sections at once and compositing in Photoshop, or whether I can get a tool that'll let me work on the whole thing all together.