4x2x2 is considered the bare minimum for a bci. Depends on it's size of course.

I was about to buy a bci and built a 5x2x2 for it last year. Then it was supposed to be for a russian tortoise, but I ended up buying two more balls instead. I used it as a bp cage for about 4 months. I was being such a n00b tho with them all together, so I built 2 smaller cages.