Another problem I see here is that at least Anim8or has problems with objects at or bigger then 1 km; even the smallest of the three most popular designs, the Stanford Torus, is bigger then this.
I'm not sure about other modelling programs though.
Scaling meshes can be done in the mesh MFC application that comes with the SDK, but this seems to remove any textures or UV coodinates on the mesh. I'm not sure if Mesh Wizard could do the job.
And Orbiter even starts to have physics problems with large vessels; I heard Urwumpe saying that when he tried to make a 30 km long vessel, it wouldn't turn at all.
EDIT:
Imagine the poly count that mesh would make.
I dread to think.
If you look at the credits before the video, you'll see that there are millions of trees, thousands of buildings, and hundereds of clouds, and that it was all generated automatically.
I couldn't imagine that working in Orbiter unless it was all load-on demand as you came close.