Small rats are definetly bigger than adult mice by quite a bit, i remember when i switched my snake over i didn't think it was going to be able to handle the small rat, but i under estimated it. he took it down like a champ.
-James-
Excellence is not a matter of chance, but a matter of choice.
I like looking at rodentpro.com when it comes to feeder sizes. They have pretty much the same info as the link Joanna gave you, but they also have pictures so you get a better idea of the size.
I think the small rat probably is "stronger" than the adult mouse simply because it is bigger, but I would trust what Deborah said about their teeth, so it sounds like it would be less likely to injure your snake.