If you have the space a 24x24x48 or bigger will be best if your cham will be spending most of his/her time in the cage. You would be ok with that cage if you free-ranged the majority of the day. So yes, you should get a bigger cage for your cham, they'll very much appreciate it!
I have a 18 X 30 X36 and if it were the only place I kept my chameleon I would have gotten something bigger. That is a little bigger than yours and I still don't believe it is large enough. My cham spends most of the day out of that cage.