I started off with an 18" cage but they grow pretty quickly..my Cham was 3 months when I bought him..and at 6 months I put him in a 48" cage..hope this helps you..Ive read a large cage for a very young Chameleon shouldnt be until he gets bigger and around 6-7 months..i dont know how true that is but took the advice and my Cham is a happy camper.
Welcome to the forums, A good size would be a 18x18 glass cage or small screen cage to start then eventually get a larger screen cage(24x24x48) for when he is an adult.
Panther male chameleons typically start their coloration as young as 3 months, sometime a bit longer, and continue to develop their color until they reach a point where they get all of their color around 1 and a 1/2 years old typically.