General rule of thumb is the enclosure should be at least 3/4 the length of the body and use good judgement on the width. Height really isn't a biggie. 12" is MORE than enough but most tubs aren't even close to that and the animals do fine. IMO a 24" tall cage for a boa is a waste of space/money if you are breeding. If its a display snake then it all depends on what you want and your wallet.