Oh, wow no one mentioned it. 10-15% of his body weight, maxing out at 150 grams. Or, about as big as he is around at his widest point. Those should be about the same. Small rat should be good, but "small" can be subjective, so try the above measurements. If the food is too small, they may not even see it as food, which is probably the problem if you're offering pinkies. And yeah, a full grown rat is too big. Do you know his weight atm? I don't know if he's a big boy of course, but he looks like it to me.