There are reasons to get smaller cages, hunting food being one. But, if you are like most of the owners on the forum, you will gain a liking to cup feeding and wont have to worry about that. I'm sure a nasty fall is also on the mind of whoever mentioned a smaller cage as well, with plenty of climbing areas and no worry about sudden fright, you shouldn't have to worry about that. My guy is 5 months old and compared to what I see here, not so big at all. He is in a 4'x2'x31" climbing area. I cup feed him and give him a lot of branches and vines to climb on. You are better off buying big, especially at 5 months old, and I agree with CBPanther. LLLReptile is a great company to buy from, that is where I buy my Chameleon package from which included, chameleon, small cage, substrate (trashed), lights, fixtures, vines and foliage. I also do all my feeder crickets/roaches/and worm purchases from them.
There are a lot of sites mentioned in the forums that sale cages, bigger for cheaper...unless you absolutely need one right this minute, I'd hold off and search the web.