I would think it would equate to around 20 gallons.

Will it fit an adult? I wouldn't see why not. People say a bp can live comfortably in a 20 gallon for their whole life. I personally junked my 20 gallon viv for a reptile basic 3x2 cage.

I am unsure about adding a second heating source. Your best bet is to email reptile basics...they are really nice and very informative. He'll steer you in the right direction.