You'd be surprised how well they can judge for themselves though. They have a lot more sense than you think.
I would advocate this much more for young chameleons that still might not be as adept at judging sizes (it's like puppies, their eyes are bigger than their stomachs!) But for adults, I've noticed that all of mine will decide when and if to go after something larger, like a full grown hormworm or large grasshopper. And if they start chewing and decide they don't want to/can't handle it, they spit it out. After all, chameleons don't want to choke any more than we do! They do a fairly good job of judging, in my opinion.