Making Landscapes of specific size

In short, I want to give myself a 5km by 5km square of landscape to carve my world out of. I want to know how to generate that specific size of land with the “Create new landscape” settings.

A longer explanation. I’m planning to make a large world, but it will be pretty linear, taking place mostly in canyons I’m gonna carve out. I’ve researched a bit and it seems adding on additional “sections” of landscape later can be wonky, and starting small then adding on would be messy. So I just want to give myself a pretty large area to work with, that I think I can fit everything inside of.I’m considering getting a relatively flat scape from WorldMachine and using that, especially if that allows me to do a specific size. But I’d like some tips. Thanks.

I still require help. I am now looking into using WorldMachine but that isn’t very straightforward either.