"I was concerned that the 5x7 is not that much bigger than 5x4 and I want an appreciable difference. I agree with 5x4 starting to lose it at 20x16, but on the other hand it can look incredibly impressive if the subject suits it (lots if detail meaning there are no large areas of contiuous tone)."
4x5=20 and 5x7=35 so it is nearly twice the square inches at 5x7. 8x10=80 square inches and is really 4 times the square inches of 4x5. It is a neat format but the film is rarer than either 4x5 or 8x10. But it is still available. I think it is too small to contact but I don't have an enlarger any more for that format. So for me, I contact 8x10 and enlarge 4x5.