The pic is definitely deceiving, she didnt look 2 ft long, a snake that length should handle larger mice no problem, just experiment a little, try increasing feeding frequency or give her more mice per feeding if u cant get adult size yet, where I get my mice they go from adult to large adult so I cant say for sure how your rodent supplier measures them, I have never wieghed my mice or rats, I just go by the width, I try to give them rodents atleast as wide as the snakes widest part, if not a little wider...
But she is definitely still small for her age I would say, and she could be a little plumper and it sure would not hurt her, like I said ball pythons are supposed to look pretty thick in the middle...