the general rule is dont feed them anything that is longer than the space between their eyes.
from what ive read, young dragons have a kink or sharp U-turn at the end of their digestive system. if them meal is too large for a dragon to fully digest it comes out partially whole. if your dragon eats a giant cricket his little system might not be able to break the whole thing down. when he tries to poop it out whole it can get stuck and serious complications can occur.
signs might include a
black beard that doesnt go away, stripes on the chest when there wer none before, straining to poop with nothing coming out(youll know their trying when the lift their tail and squirm around)... if your dragon is exibiting more than one of these signs, you may look into a reptile vet in your area and consult him.
hope everything is ok.
P.S. if your having trouble finding a store that sells "medium" crickets(1/2-3/4 of an inch), you can buy them online here:
http://www.mulberryfarms.com/cricket.htm
i just bought 1000 4week olds(medium) for under $30 shipped to my house. that would have cost me over $100 at freaking petsmart :lol: ...