General rule of thumb is no bigger than the biggest part of the snake, but sometimes with nothing else available I have fed something very slightly bigger than the thickest part of my girl. May take it a bit longer to eat, but mainly if it's too big the snake won't eat it. Still, people compare sizes differently so you saying he's nearly as wide as a small fuzzie and me saying it could be quite different. One good thing to go by is the weight of a prey item if you have a scale.