So long as the food item isn't alot larger than the thickest part of the snake your fine. Most people try to keep it to that size, or 10 - 15% of the body weight of the snake alternatively (if your feeding more than one prey item, and it's smaller). I generally believe if in doubt, go with a smaller sized meal. You'll know if it's too large for your snake because they will regurge. Something you generally don't want to happen.
In regards to digesting. Larger meals can take a little longer, however his defecation should remain unchanged for the most part. Snakes are pretty good with metabolism.