JB,

How big could you build a shape like this, assuming that the walls are
covered in Auralex or the eqivelant, before you would start to have a
problem?

It's not at what *size* you need more than thin foam on the walls, it's what
*frequencies* will be sounding in the room. If it's for voice only, the
requirements for a booth are not as stringent as when recording a guitar or
bass amp or string bass player or drum set.

--Ethan