For the life of me I can't understand why beginners always go for the biggest warclub they can find. I am coaching 9u rec ball. I have already had to take the 29/18 bats we have and hide them. Most are having trouble with the 27 and 28's the organization supplied us. I bought a 24/15 for my DGD and half the team is using it after 2 practices, not by their choice, mine. I was always told that you should control the bat, not have the bat control you.
So is there a rule of thumb for bat size? MY dd is 11, 5'2" weighs 85lbs and is currently using 31/21. I feel this is too heavy for her and had her use a lighter bat last night, 30/19 and she hits a nice line drive over 3rd base for a stand up dbl. I ask her if she is ready to put the bigger bat away for a year or so. Nope she wants the bigger bat.
Parents tend to get bats for kids that are too big. Maybe they think they'll grow into them. Kids get attached to THEIR bat and want to use that one bat no matter what. Drives me crazy! Maybe they were able to get a great hit ONCE and so they believe the bat has some kind of good juju or something.
Here is one guideline for sizing bats. It seems close, but according to this, My 10yo should be swinging a 30" bat where I just bought her a 29" bat because I could see she was having trouble handling the 30". bat was not too heavy, just too long. I tell parents that swinging a bat that's too long is like trying to swing an oar at the ball. Bat Sizing Guide - The Batter's Box
The sizing guide for length is pretty accurate. But nobody has mentioned drop. I think that is more important at the younger ages when they're building their mechanics. There is a big difference between a 30" -12, and a 30" -10. If the kid has the arm strength and the mechanics, then they can most likely handle the additional weight.
I agree younger kids typically swing bats too long and too heavy. It's a huge problems when kids are trying to learn to swing properly. But I'm not a big fan of some drop 12s, against good young pitching they can't "stand up to" the ball and you do lose some power/distance. And that's frustrating when kids are making good tee and soft-toss swings but then get overwhelmed against a 50mph 10U fastball. Best bet is to take a slightly smaller bat at -10 I think at the younger ages.
A kid on my 8U team had a bat that their aunt used in college. I could not convince the parents or the kid to use a smaller bat, they had to use the adult sized bat that aunt Ruth hit a home run with in the 70's.
Mine is 9 years old. Around 4''10" and weighs about 80ish. She handles a 30" 20oz Synergy (few years old...got it on sale) no problem. If they can swing it and you can get a good price on it I think the ball jumps off the good bats like this Synergy with a lot more energy than the -12 metal bats...a lot more. I observe this both by watching the kids and personally hitting grounders with both types of bats. When arm length is taken into account I think it is very hard for any kid to even cover middle/out with a 28" bat or less.
Inspite of all that until the girl develops a little strength and a swing that somewhat stays connected and is not just initiated and finished with arms it is better to go lighter and let them work on the swing.
I had a girl on my 10u rec team show up with a 33/24 that she had used last fall and spring. Apparently she was having some issues at the plate. She said she would only use that bat because other bats made her hands sting. I did manage to get her to switch bats.