Smallest I have is a 32qt and my males will reside there and females in 41qt. I dont know when I would need larger for a BP but I plan on 74qt racks for my RTBs.... I do know of a couple prominent boa guys who house 4-5foot boas in 41qt. I would say play it by ear. Some snakes want space, my female boa would probably rather be I a shoebox rather than a 74.