Sorry, that doesn't make any sense.
Enclosure size has nothing to do with how big they grow. Nothing at all.
a 20g long is perfectly fine for a full grown MALE bp. I would never put a male bp in anything bigger. A full grown FEMALE bp would go in a 30g long, nothing more.
A 55g is the most useless tank for a BP. It is far too tall and has about 10 more square inches of space than a 30g long.