Although the general opinion seems to be to start off with a cage about 18x18x20 or smaller for young Chams, I have found that if I create a cup feeder (see DIY search word milk), and have free roaming crawling feeders and flying blue bottle flies, that I can start off with an adult size cage with a base of 2 feet by 2 feet, and 3 or 4 feet tall. By putting a small Cham into a large cage, I think it is of great benifit for hiding, stress reduction, and creating more options for the Cham to move about and regulate heat.
All but one of my Chams when purchased were 4 months or older, and they all started in large cages.
To be sure the Cham is eating in a large or small cage you have 3 options:
1) You can weigh the Cham on a gram scale.
2) You can count feeders going into the cage.
3) You can "measure" how much droppings are coming from the Cham.
#1 is not the most enjoyable option for a Cham-I don't handle mine unless I have to.
#2 is not something I have time or patience for. I don't like to create my own stress-who figured out a Cham at any certain age will eat so many crickets? Sometimes we work ourselves into a stress mode, don't we?

#3 is something I do when cleaning the cage. I also check the "quality" of the droppings. Crap/Frass/Poo/Poop/Elimination Product, whatever!
I totaled the costs of "double caging" and figured I have saved at least $100.00 for the extra setup. Not only the cost of the cage, but the light fixtures and bubs, the plants and climbing surfaces all get upgraded every time I do a new cage-That is part of the FUN isn't it?
Hope this helps
Nick