Generally, the prey item should be at least the width of the width of the snake, and no more than two times the width of the snake at its widest part. Also, if you know the weight of the snake that helps a lot, that way you can follow the Munson Feeding Plan.
-When they're on single pinks (2-3g), I feed every 5-6 days. (Snake = 4-15g)
-Double pinks (3g x 2) every 5-6 days. (Snake = 16-23g)
-Small fuzzies (5-7g) every 6-7 days. (Snake = 24-30g)
-Regular fuzzies (7-9g) every 6-7 days (Snake = 30-50g)
-Hoppers (9-12g) every 6-7 days (Snake = 51-90g)
-Weaned (14-20g) every 7 days (Snake = 91-170g)
-Adult (20-30g) every 7-x days (Snake = 170+) See below.
Note: Adult females are fed more frequently than adult males (especially following brumation). Adult females are fed every 7-12 days; adult males are fed every 11-14 days.
This is by no means scientific, and not all corns will cooperate 100% with the schedule. The weight ranges I gave for the prey and snakes are approximate.