Am I feeding my ball python the correct size?

[nikkubus]

So I'm at Petco right now because there isn't reptile stores near me that sell frozen, and turns out I am feeding her large mice, the bag label says adult mouse 18g and the next size up would be small rat at 40g. I'm unsure if I should just get the small rat or keep with the adult mouse. There's no bigger mouse than the 18g one.

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I'm not sure what all petco offers, but ~25g would be perfect. "small rat" are generally way too big, do they have rat pups?