i'm 5 ft 1 inch and I swore by medium sized bags for so long, but I tried on the teen size and now I'm selling my larger bags to get a teen classic box instead.
I'm 5'4 and I have two mediums. Med is already too small to fit my belongings lol. the bag fits me better when i shorten the straps, so the bag is under arm, not letting it sit below hips!
I'm 5'0 and the medium looked gigantic and formal on me. The teen size was nice, but the strap even on the shortest setting was too long. It looked weird crossbody and worn on the shoulder with how long it was.. Unfortunate as I think the teen size was perfect otherwise!