I normally go by their girth size and feed something up to 1.5 times the size of that. Every so often I'll feed slightly larger if that's what I happen to have but overall, corns will try to eat just about anything so I don't like pushing it too much. I'd use up what you have left of the smaller ones and then switch to the next size up. If they act hungry, I'll give them 2 without an issue. You'll know if they start getting fat and that's what you want to avoid.