Many of the online sights for purchasing crickets have a diagram of the actual sizes, I dont know about panthers but he is still young at 4 months. Check the sites out first, you can go with the size between the eyes of your chameleon or a small step up. If you buy a lot remember they will get bigger in a week so maybe buy a size smaller and feed more until the others grow to size, especially if you are going to buy 250/500+...just a thought that crossed my mind when I was going to buy some becuz more is cheaper but it may be too many. If you are not going to buy a lot go to a bait store (see if they have them separated in size), shipping makes everything expensive.
kire