sorry for the late reply -- I guess this board doesn't see much action.

My rule for frogs is that if it looks like it might possibly fit in the animals mouth and moves, it's potential food. I once heard of a Bullfrog found with a chipmonk in its stomach... now that's a bit of an extreme and it wasn't a huge chipmonk but it gives you an idea of the possiblities. I'm kind of making these numbers up but I'd say if it's at least the same width as the head of the frog and twice as long as it is wide then it's pretty safe. If your fish that got eaten was already that big then you may want to find someone who has some experience with fish/frog compatability for a better reply