You're the one with a Ph.D I just have a P.H.D. but I save it for the ladies... anyways...
I had to get some pots for a few plants and it was either a 2gal or 3gal pot... so to save space I went with the 2gal. I had a plant go to about 3.5 feet tall and the roots didn't take up all the room in the pot! I now aim to keep it @-~ 2.5' tall. Any bigger go with a 3gal pot.
All this pot talk... it's about time