Depends on what kind you want. They need a lot of vertical space. Mine was maybe 8" including the tail and he was in a screen enclosure (which is important they need major ventilation + high humidity) the cage was slightly larger than a 20 gallon tank.. but a grown chameleon would need more room.

Chams are insectivores. Mine liked crickets, superworms, mealworms, and whatever other crawly things (I'm sure roaches would have gone over well too) his favorite thing was flies! Regular old house flies. He'd shoot his tongue and catch them in mid air.

An important thing about chams is they do not recognize standing water. You need to have a dripper drop water on some leaves (I had a ficus tree in his cage) and they will lick the water from the leaves. I sprayed the cage several times a day too.