1' 6" should do in height, and if space is no problem I would give the BP 4ft length. I've got my male in a 4ft long container, and he has a tiny amount of space between his nose and the side of it when he stretches out from side to side (long way). so he is only teeny bit smaller than 4ft. IMHO I like to give them atleast the length of their body. That is if space is not a problem.
Neptune, Zion, Enigma,