Yes, food sizing is important. Select a mouse/rat that is appropriate for the size of the snake. The mid-section of the snake is where the largest girth (diameter) is. The prey should equal this, at least that's how we do things here anyways. Just use that as a guage. Our Kingsnake takes down prey a little larger than this diameter easily on a regular basis. Our RTB we actually have to give smaller than usual prey.

We once had a Bullsnake at work we saw trying to wolf down a rabbit that was about 4 or 5 times waaay to big for it. It worked and worked trying to get it down and eventually gave up 'cuz later the snake was gone and the rabbit was in the same location - abiet regurgitated. If the snake senses it just ain't gonna' work - that's what happens.