If her age and weight are correct, then she has been grossly underfed. Most BPs graduate beyond the size of a large mouse before their first year and that's eating weekly. I would start by trying to switch her to rats, which are easier to get in larger sizes (most adult BPs eat small or medium rats).
If you just got her, work on her husbandry first and let her settle for a few days or a week. Then feed her a large mouse which she has been having to make sure she will eat for you. Then the next week, go get a rat pup (about the size of a large mouse) and see if she will take it. Once she's on rats, just feed her whatever size rat is about the width of her widest part and hopefully she will start growing for you.
If she was taking frozen/thawed mice, you can try a F/T rat, though you may have to try live first to get her to switch.
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