Now let me warn you if you have a particular fondness for mice or rats, snakes are not for you. The size of the mouse or rat you need to feed your snake, really depends on the size of your snake, because if you have a little baby snake you don’t want to give it a massive rat.
Most baby snakes will eat pinkies, which are basically just mice that are about one day old and have no hair. When your snake gets bigger you are going to need bigger mice or rats. A generally ok rule is don’t give you snake anything bigger that the thickest part of the snake.
You just need to change it every day and you are pretty much fine. Some people say you shouldn’t put the water bowl in the hot end because there is more bacteria in the hot end.