verboten: forbidden; prohibited by dictate
Anyway, I have some pretty big spaces in the L4D campaign I'm working on, but not much bigger, if any, than what valve uses. Experimentation is key here. As fry_ said, optimization is really important. Try
this is you haven't seen it before... In my experience, the fog doesn't work to optimize the way you want it too. Still lots of glitches w/ things appearing beyond the Far Z Clip plane. See
here and a bunch of other places... Echo51 is also right. You can get away with some pretty big stuff, but the more crap you pack into it, the nastier it's gonna be. Good luck!