A good demension estimate would be something around 3ft L x 1.5ft W x 1ft T.
That would be a perfect ball python cage in my opinion. :]
2x2x1 wouldn't be terrible either. A lot of adult ball pythons live in 32qts happily, and those are 2ft deep and 1ft wide. :]
If it concerns you, you could always divide a 5x2x1 cage into two 2.5x2x1.