My snake was born in June, so not very big. I haven't weighed him yet b/c he's been eating very regularly and so I haven't worried about it. I probably should be more responsible about his measurements, but I've just been eyeballing his prey size and things have gone well.
He's typically eating every 5-6 days, so I could keep the mouse and feed him again in 3(?) days? I don't have any mouse food, however, so I'm not sure how long mice can go without food. And I don't want the little guy to suffer needlessly either.
ETA: I could always return him to the pet store. I don't need my money back or anything, I just wasn't sure if there was an easy way to somehow use him for Herman's next feeding.