She does sound a bit small but then all snakes grow at different rates. The fact that the pet store was feeding the poor thing a tiny pinky mouse every 2 weeks is likely why she's the size she is. Our feeding routine here is every 5 days until they reach 150 to 200 grams, then once every 7 days. She's likely quite able to take a young adult mouse (not a huge breeder mouse, just a nice young adult/recently weaned size).

Just remember to not rush anything, just a nice steady increase in prey size and you'll see a good and healthy weight gain.