abrisley is right on. Depending on the size of the player and the position they play, the glove size they use will vary from 12"-13". Most pitchers will use a 12" to keep the glove weight down while pitching and most infielders will use the same size. Outfielders will usually use something a little bigger like a 12.5"
Take a look at your glove. Inside it is a loop for your thumb and there should also be a loop for your little finger. As long as those are adjusted so the girl's fingers fit in them, they can go a lot larger than they think they can.
Of course you wouldn't want an 11 inch glove for a 12 inch ball, but parents start out thinking that a glove should fit like, well, a glove. Look and see how the 12 inch ball fits in the pocket of the glove you are considering. And decide if you want a weaved pocket or an open one (Shaped like #). I prefer an #, because I can see through it.
Decide what you can spend and if you want to take a long time to break the glove in. Some are easier to break in than others.
Also, make certain that it is a softball glove and not a baseball glove. I am guessing that most larger gloves are softball.
Another important thing to look at is the pocket (web). Some are bigger than others.
A beginning girl does not need an expensive glove. I am thinking that $80.00 would get you a decent one off the shelf.
Another thing to tell the dads. DO NOT play catch with their DD's glove to try and speed up the break-in process. Their fingers will stretch the finger holes out on the glove. Let the glove stay small to fit their hands. I know for a while one of the glove makers was selling gloves geared towards the smaller female hand.
Amy's is a great summary. I find too many girls with gloves that are too big for them. If the girls are going to become serious ball players, they will also start to realize different gloves have different "feels" to them. e.g. my DD like Mizunos. Has tried others but likes the feel, fit and function of the Mizunos.