I've got mine in a 24" cube - It looks grossly over huge with him/her just sitting there during the day - but at night he/she likes cruising the cage and playing in the plants and everything - uncoiled GTPs are surprisingly big.
It's pretty old, but this is the only picture I could find of the cage right off:
BTW - the more perches the better it can thermo regulate - also if you are getting a baby - then it does need to be kept in a smaller cage for a while. I didn't see what age you were looking to get.