Ideally, you want the largest you can reasonably afford, but (IMO) a minimum of 24x24x48.
Personally, I'm building one 30x30x48 because that's the space I've been allotted on the Missus' plant table. The table is 30" tall, so another 36" (one foot from top of cage) puts the chameleon at 5'-6", which is just about my eye level. ?