This is what I do, I take a piece of string and put around the widest part of snakes body then I hold where the strings meet and take it off bp and then cut it. Then I take the string with me to where I get rodents and wrap it around the prey and whatever one it fits best around is the one I get. A tiny bit bigger won't hurt anything if the mediums happen to not quite make the ends of the string meet. You may be surprised though, my killer bee I bought a month ago was being fed small mice which I knew were to small so I took the string with me and mediums were even to small, large was perfect.

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