My DD used a 11.75" Mizuno glove for both 12U years. She is taller than most for her age and her usual positions are P, 1st & 3rd. We got her a new glove last week. Bumped her up to a 12" Mizuno.
For most of my infielders, in both high school and travel, they use Akadema ABH 99 11.5" gloves. We get them from the Akadema rep for $20 to $25 bucks each, so we usually buy 5 or 10 each time. They are a little prone to string breakage, but only after tons of use. Most of our infielders are getting 3 or 4 buckets a day nearly every day of the week in season. For $20 bucks if it only lasts 1 season who cares? Buy another one.
The mold is perfect right out of the box, I don't even put a ball in it. Don't have to worry about the ball getting lost in the pocket either, so transfer times increase. On backhands, I don't even close the glove, it just sticks in the pocket.
I can't say enough good things about this glove.
Forgot that we use them for most of our infielders in 12U as well.
Experienced middle infielders always seem to like small gloves your 1B and outfielders probably want something a little bigger, also at this age have to remember girls that have not had any kind of growth spurt might be really small.
I think 12 to 13 is average, my DD went from a 11.5 @12YO she grew 6 inches and uses a 12.5 Mizuno now that she loves, I can't believe how much these new gloves are already broken in she had no issues from Day 1.