To handle objects of absurd size you have to have an engine capable of generalizing terrain LOD rendering to models, and probably some method of handling insane scale differences. I'm sure the FS engine could handle fairly large models simply by modern graphics cards brute forcing it.
It is possible, however, to render absurd scales because you never require high precision at extreme distances, so if your galaxy was being rendered with a unit precision of ten light years it'd be perfectly fine as long as it was really far away, since the andromeda galaxy moving 10 light years isn't even visible. Unfortunately no modern 3D graphics engine bothers to do this. Yet.