It's pretty big, actually. There are six sectors, each with quite a few things in them. I'd say about the same as a decent sized city. Bigger than the Garrison, but not as big as like... Vegas.
[He tried to think of the biggest city close to the Garrison. That was it. And then maybe something else that was close by that would be around the same size as the dome.]
I guess maybe about the same as Caliente, if you've ever been there.