Leo, it needs to be big enough for a 12inch ball to fit in the pocket. Check the pocket size.
Also, look for the loops for the little finger and thumb to fit in. You can wear a pretty big glove if your fingers fit in the loops. They are adjustable.
Recently, I have been using a Worth Freedom glove. It is priced reasonably and is easy to break in. The Worth Liberty is more pricey.
With the glove it depends on where she plays. An infielder wants a glove that fits like a glove so the ball doesn't get lost in it and allows for a quick ball to hand transfer. My 17 year third baseman on my high school team uses a 11.5 inch glove.
An outfielder makes alot of running and diving plays and the bigger the glove sometimes is better. A little extra leather may make a difference in whether that long fly ball she just ran down gets caught or not. My DD has a 13 inch glove in her bag because she plays CF.
Most importantly find a glove that is adjustable and fits her hand properly. You don't want something loose that she has to keep shoving back on her hand, and you don't want to have put on batting gloves or extra padding in there to make it fit. Always keep a ball in the pocket to keep it formed, and regularly clean and tighten the strings so it lasts longer. I see many kids in our camps that have loose strings.
Hope this helps, but remember.....bigger is not necessarily always better.
A big glove is a crutch for many girls. They dont need it, it actually holds them back.
Infield, small gloves with very shallow pockets for SS and 2b to release the ball quickly. Little larger with deeper pocket is oK for 3B, speed not as critical. If you dont squeeze these gloves, the ball will drop right out into your other hand, which is what you want.
OF, deep pockets that hold on to the ball by itself in awkward sliding, rolling, tumbling catches.
A problem to me, is the lack of quality fastpitch softball gloves (made for smaller female hands) , especially with open webbing pockets. The open webbing is what you really want in the infield to keep from flinging a glove full of dirt up into your face, or getting a handful of dirt along with the ball. Choices are too limited, so a lot of girls use baseball gloves. I have game pics of my DD that show dirt pouring out of her open web glove as she fields balls.