There's quite a bit of scope on the velcro. My medium weight 15.2 horse always had the large ones. Not sure if there is a sizing guide on their website or not but if you give them a call with her measurements you'll find them ever so helpful.
Mine are large, I bought them for a middle weight 15 hands horse, I've used them on all sorts of sizes, they would fit pretty much anything from 14.2 I'd say. As above, there is plenty of room for adjustment with the velcro. The small ones might be too short I think.
I use a small on a 14.2 and they are just right I would think they would come up short on a 15.2 but you can get a medium on their show stand or if you ring up as most places only advertise small or large