Think about what size that you would like to record drums in and build it to that spec. The reason why I say this is because you will eventually want to use this space for other pourposes. My vocal booth is 10'x 7'x7'6" high.
Allways think bigger, you will be more comfortable and wont want more as soon as its done.
Hi - Since a vocal booth is usually made to be a very dead space, the size doesn't really matter, acousticaly, as long as there's room for the number of singers and mics/stands you expect to use in it.
Cheers, RD