You will see Limbs or other things pprotrude sometimes while its in the neck area... As you said you want to feed the thickness of the Body. A Ball Python has a small head so do not go bigger than the body. Snakes will in fact try to eat larger prey and it can cause problems thats why we use a reference.
I had a Ball Python basically choke because I was use to feeding Burms and it kept trying to start over, it scared the hell out of me and I've fed correctly and smaller since.
They will also regurgitate a prey item thats to big, and you want to avoid that at all costs.
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