Hard to say. It looks like a lot of it will be underground, which removes the possibility for fast-travelling, therefore won't have map markers. So, I don't think it'll be "big" in covering a lot of surface area, but it'll probably be deep like the Vaults.
Oh, and "Big Empty" is a bastardisation of "Big Mt" (Big Mountain), though it isn't clear exactly to which real-world mountain it relates. We know it's a crater, so I'd guess the rim would be roughly the size of the base of Black Mountain.
I would hope it's larger than black mountain. Point lookout was about a fourth of the CW. Hopefully the big empty is about as big, black mountain, base included is only about a sixteenth of the playable MW.
But like you said, if there are many areas as big as vaults underground, it would eliminate the need for a lot of space, but even judging by the previews, the Big Empty looks quite large.