Go to a reputable athletic store that has someone who can help you select a bat in your price range. If you like a bat, but the price seems high, check it out on eBay.

Hold the bat straight up on the floor and the knob should hit directly at the hip bone. (For length) The player should be able to hold the bat with her strong hand (right hand if right handed) straight out in front of her body for 60 seconds without her wrist shaking or giving. (For weight) Try out a few until you find the heaviest one she can hold properly for one minute.