I bought my set up from Kammerflage Kreations and they recommended a 18 X 30 X 36 for a 4-5 month old and that worked out fine for me and he is still living in it as an adult. They keep all their adults in them. It is decently roomy but I also have a free range area and a large outdoor cage. If this were the only cage I had, I would probably have something bigger indoors.
Maybe its just mine, so it won't necesarily be the same for yours, but I've always seemed to have small panthers until I'd move them to a really large enclosure. It seems as though the added excercise of roaming a larger space helps them metabolize their food better and grow at a more normal pace? Because I'd have a 9 month old panther that was still at 30g and comparable to a normal 3-4 month old. Now he's 1 year old and he's nearly trippled at around 80-something g. All seemingly due to the much larger cages, because no other variables changed (food, amount of food, lighting, temps, etc.) So I don't know if my observations have any validity, but I tend to think young chameleons would do just fine (or better?) in one of the 4x2x2 cages.
I have had some grow large young, and some wait until a year to grow-I have not noticed cage size limiting their growth like you do see in fish. If I have a really good hunter I don't usually have an issue moving them up in cage size.
Im building a 5 foot wide x 6 1/2 feet tall cage, in my opinion has with any other animal, the bigger the better. These things live in the the open wild. Has long has you put plenty of plants, vines and leaves everywhere so he can hide if he feels like it, have fun with wiyh tat, i know im having fun.
oh, and tie a large empty yogurt container to his main tree, take a stick that goes into it that the cham can climb on, and put the crickets in there so he does not have a hard time catching them due to the size of the cage...