[Natural Sound]

We've done it with about 100 sq ft of space.
The walls are double framed. The ceiling is about 12 ft and is the bass trap. There are no parallel walls. The room is medium dead.

A room of this size should not be ambient as it will sound pretty much like a shower stall. And it will sound that way forever.

If you want to capture any real useable ambience you need a really large space. I would say at least 500 sq ft.

So if it's going to be a small space, make it deadish, practice with all types of micing techniques and use outboard verb.