I like the idea of you doing a contest like this, the whole thing is pretty much gonna have to be on a trust basis, but then again, you are talking to a bunch of "fishermen" and we all have fishin tales. And with the technology of computers these days it would be easy for anyone to super-impose any image at a smaller size to lay next to their fish. You may want to have them lay the fish on top of some sort of document or maybe a notebook which is 8.5X11 inches and even lay a tape measure next to the notebook.
I would like to do the contest thing, but down here in south louisiana the crappie don't get that big, Been fishing here all my life and i caught the biggest crappie I have ever caught a few weeks ago and it was only 12.5 inches long. So good luck to all that participate in it if it happens.
You can't fish with a hung line!