They can "handle" bigger prey than you would think, but it isn't good for them to do it regularly (or really at all). It usually takes them extra time to digest and extra energy which can be a big problem with a snake with any other health issues or under severe stress.
Mystic will probably be just fine as long as you get the right sizes from now on. The biggest issue is usually fully swallowing and regurgitating the meal a few days later. Spitting it out before getting it down is not nearly as rough on them. The week off while you get rat pups may even give him a bit of a break from all that digesting he did before.
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