What other fish do you have in your tank? I usually try to guesstimate the size, and make sure they can't easily fit in the biggest fish's mouth. However, if you have some aggressive fish in there, that doesn't always work.
Another thing you need to be careful of is keeping them in nets too long, even if you do have some big fish that could possibly eat them. I'm guessing you mean breeder nets, or do you have a different setup?