If you don't have a scale to weigh the prey, there are other ways to determine the correct size. You can use one of those cloth tape measures like you can find in the sewing section of Wal-mart to measure the circumference of your snake at it's thickest part, then use it to measure the circumference of the prey, which should be about the same size. If you can't get one of those tapes, you can always wrap a strip of paper around your snake and make a mark where the paper overlaps, and then use that to measure the prey...