Depends on the adult. Most males can live in a 20Long or at most a 30 gal breeder tank, but some females need increased space.

Whatever tank you use, make sure it has more length and width than height! BPs don't need ANY more than a 12" tall home. 30 gal Breeder tanks are low and wide, thus making them a great choice for most adult BPs, and 40 gal breeder tanks can be used for the big girls.

I may be building a rack right now for my BPs, but I'm still mainly a tank person. I'll still have two BPs in tanks because they prefer it, and my tubs for the rack are 90qt - same L&W as the 41, but 12" tall. I prefer giving my snakes some head room as well as the room for greenery.