In the FAQ topic, there is a section that discusses feeder sizes. Basically, you want to be feeding your snake food that is aproximately 1.5X as wide as the widest part of the snake's body. Any wider and digestion could be an issue. Any smaller, and the meal won't be as satisfying nor will it contain the appropriate amount of calories and nutrients to keep your snake healthy.
Most of the basic care questions can be answered by reading the thread labled as "Basic Care and Hubsndry FAQ". It is very informative and provides most of the information you will need to keep a healthy snake.
Good luck and have fun with your pal...