A 20 gallon long could actually be too big for a baby BP, but you can clutter it with extra hides and crumpled newspaper to fill the space and make him/her feel more secure. A 40-gal is large enough for the majority of adult BPs to live in. I kept a 2400g female in one for a few months, until I switched to racks, with no issue. Just do a search for some of the tank modifications suggested on here for keeping in humidity, supplemental heating, etc.