From what I know, suitable size of prey is tied to the thickest width of the body, not primarily the length. A general rule of thumb that I have read is that the prey should be no more than 1 and a 1/2 times the thickest part of the snake.
However if husbandry is perfect and the snake is healthy then you can get away with a little larger. I have trouble buying small fuzzies for my 7 month old, she is 22.5 inches long. The 'fuzzies' I get are completely furred and might be better considered small hoppers. However, as she has a good hot spot at 33 degrees c she digests these easily and always defecates three days after ingestion. She eats one of these every six days. I will probably move her up to 2 of these every 7 days in a month or so.
On the other hand 15 inches does sound on the small side for eating fuzzies. My corn was eating two pinkies at that age. But if he is digesting them properly it can only be good for him as they are more nutritious!
