Depending on where you are getting them from there are some gaps between sizes where one is too small and the next one up is too large. You should definitely get a digital scale just so you can monitor your snakes growth and health. If he ever goes off feed you'll want to know if he starts loseing weight and how much. For feeders you can also look at the girth of the snake at it's widest point and select a prey item the same size. You may have to feed two smaller prey if your snake isn't quite big enough to go to the next size. Switching to rats is beneficial because they contain more protein and fat than mice.