Weaned rats are generally a little larger than an adult mouse. Here's a pretty good chart on feeder sizes http://ball-pythons.net/forums/showt...mparison-Chart
Live VS Frozen/Thawed? They are both fine. I feed f/t because it's more convenient for me and all my snakes will take f/t. I've had to feed live in the past, though. It comes down to: what will they eat, and is it convenient for you?
Rats VS Mice is another can of worms. Rats have a slightly higher nutritional value, but since no one really knows what a Ball Pythons nutritional requirements are, it's doesn't really matter. Rats are more convenient to feed, because, as the snake grows you won't have to feed multiple smaller prey items per feeding.