You could probably even go up to a weaned rat. From the looks of the video, the hopper is just about the size of your BP's neck--the smallest part of her body (except the tail).
I like my snakes to have a noticeable lump in their bellies. I have a couple other species that will take prey that is twice the girth of their thickest body portion. With my balls, I shoot for about 125-150% of the thickest body portion.