I always get a kick out of parents who like to brag about the size bat their DD swings....."my DD swings a 34/26".....it is like a bunch of rednecks sitting around a campfire, consuming adult beverages, telling stories about the biggest fish they ever caught. As a coach I really don't care what size bat your DD swings, all I care about is results. But I can tell you from experience that if you let a players dad pick out her bat it is probably more bat than she needs. Get her batting coach or head coach to recommend a size/weight. Let her hit some different weight bats, brands at practice while she has access to teammates bats.
The pitch speed also plays a big role in what size bat a player can use. We just picked up a B-level TB player on our team who had to go buy a new bat because she could not get around on A-level pitching with her old bat. There is a big difference hitting 52 MPH pitching vs. 62 MPH pitching.