It's too full if they vomit. My chicks gorge themselves all the time. I wouldn't recommend it on a regular basis, but they will be okay. After gorging they usually go into food coma. lol It will always get larger after eating, but when they've stuffed themselves they'll twist their neck and gasp in an effort to push more food down (they'll do this if they eat too fast also). When it's REALLY full you can see straight through the crop and look at everything they just ate. EEEEW... but fun.
A full crop isn't a problem as long as things keep moving. Offer fresh, cool water to your birds at all times and crop binding should not be a concern. If they do get crop bound, you can usually get things moving again simply by firmly massaging the crop. If you're worried that it's too full, just gently massage it. Also, they will probably gasp when you massage. You may be pushing against the trachea or lungs... no reason to worry, just give them breaks in between massaging. Also also... provide grit, they'll need it to digest the corn cobs.