Others may disagree, but I think you'll want something longer than that. Length is better than height for GTP enclosures. You want to provide a horizontal thermal gradient, so they can get to cooler spots away from the heat source without having to come down from the security of the uppermost perch.
36" long by 24" x 24" for an adult GTP.
Totally agree brotha couldn't have said it better.
I roll with a 3'x2'x2'.
Mike
"They are biting out of excessive fear, not because they are mean. Mean is a human emotion and serves no purpose in the animal kingdom" - Kevin McCurley-The Complete Ball Python