Minimally for an adult I'd say 36"x18"x18", but only if you plan to have your lights (except the UVB) outside the enclosure. The 4'x2'x2' is convenient because you can make it all out of 1 piece of 4'x8' plywood though. I would definitely say that the 4'x2'x2' is better if you plan to have your lighting mounted inside the enclosure, which kinda turns the usable safe space in the enclosure to about 4' long x 2' deep x 18" tall anyway.