She won't choke. The worst that could happen is that you feed a rat too large and she regurgitates. It sucks for the snake when that happens, but it doesn't kill the animal. Again, that's worst case scenario. I never like to feed as big as possible, I like the prey item to be manageable for the snake but still large enough to add weight to your snake with each feeding. I can tell you that your snake can absolutely handle a rat pup though.
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