Um yes, that's a bit of a problem because sometimes (no often) noones too sure, especially if it's something a bit tricky to identify. Planet catfish sometimes has max size... Gold nuggets don't get huge, 6 inches max I think except one type that goes to nine. Avoid common and gibbiceps like the plague. Lookat catfish of the month profiles
It's probably easier to see what you can get, and then see how big it is. On the bright side, a good big example of something attractive like a 12 inch L-4 would be easy to sell