Try this link and see if this helps. Usually wherever you go to buy a bat, they have this chart available.
http://www.nebaseball.com/bat_size_chart.htm

also it isn't just the size that matters, you need to make sure you check wt as well. when the child swings the bat he/she must have complete control with a direct motion forward. If it bobbles in any way... up...down... looking like waves... whatever.. the player is not getting a controlled swing and needs to decrease the wt of the bat. Also you can put the handle of the bat under the arm of the child and have then place bat out to side of body with are straight out in 90 degree angle. The bat should be no longer than 3 inches longer than the finger tips.

I hope this helps.

Coach Barb

This is how to check and see if the weight is correct. Have her put the bat in her strong hand, palm up and have her grip the bat at the knob with one hand. Have her put the bat out in front of her parallel to the groud. Then have her rotate her wrist clockwise if she's left handed and counter clockwise if she's right handed. If she struggles at all, the bat is to heavy.
She ought to be swinging a -13 (Rawling Instinct) or -12.5 (Rawlings Velocity or Mizuno Blaze). You may e-mail me at rpiening@ifungo.com, if you have any other questions.

There are charts you can look at, at a local sports store. You need to make sure she swings the bat before you go off and buy her one. Normally a persons bat size that fits them comes up to about there waist. I swing a 34,27 ya its pretty big but it suits me. hope this helps.

Have her hold a bat in her dominate hand and extented her arm if her arm shakes its to havey for her

Shoulldn't she be playing on an 8U team if she is seven years old?

you should put the bat in the childs hand and make her hold the bat paralell to the groend and make her turn the bat clockwise and if she has troubles then the bat is to heavy and to long BUT SHE NEEDS TO BE ON AN 8U TEAM ANYWAYS!!!