Trust us when we say a 20 gallon is big enough for now.I would even go so far as to tell you to get a 28 qt tub for him as it's easier to control.
Bp's are secretive, and for the sake of rehabilitation, will benefit from having a small enclosure that's easy to clean, and keep at proper levels.
Becky already went over that, and I will back her up as well. BP's do not need a ton of open space. A 20 gallon will do quite well.