I used to be sacred about size of food, especially since I started out with Corns... but noticed my daughters corn would take down two pinkies every time, so I upped it to a fuzzy... It looked like a dog trying to eat a soccer ball... well it did and was happier for it. Obviously don't want to go huge, but increase the size by small margins until it is the "right" size.
I got wingling mice, which are like hoppers, which are about the size of his thicest girth, but maybe slightly bigger, but not a lot. I know he could down it, im starting with the smallest mouse out of the ten I got