Believe it or not, I've successfully kept American bullfrogs with res's, some res's ten inches or so. Trick is allowing for a place the frog can hide, since bf's usually go in water at night while the res's sleep.
I've experimented, believe me, and had raised bf's with turtles at least three times.
Yet it depens on one factor, whether you have fish or frogs: You must allow the prey a way of escape, but of course, not out of the tank.
Right now I have three 5" res's and two 4" yb's in a 125gal pond with four sunsfish, eight large goldfish; I have an 8" oscar and on occasion, bf's I turned loose in the yard.
Gist of the matter,like someone else said, is the size of your tank. I'll tell you one thing though. If you get a large enough of a bf, the frogs won't touch it! They hate the smell.