Checked on some of the little girl's clothes just now. Medium-7/8, Large-10-12, Extra Large 14/16 so far anyways! For juniors do you mean adult size like highschool? S-4/6, M-8-10, L-12-14 so says walmart...
I really only know what's on the shirt itself (see third photo), as I'm not a girl. The shirt was bought for my sister, who's a freshman in high school, but apparently she doesn't like Angry Birds.