The appropriate size is 10-15% of the snakes body weight. If you have a 100 gram ball python you can feed it rat fuzzies.
If you start feeding mice, expect to be feeding mice to it for the rest of it's life.
This is realy not true at all. We have started some off on mice and have never had anyone try to stay on mice. We breed both rats and mice and all of our balls will eat either or.