My problem is I'm a big-frame woman (5'5", 200 lbs) and I think I'd look weird sitting on one. Plus, most of the mustangs I've come across were just under 15hh. Not exactly the right size for me.
Seriously? I'm 5'8 and weight a little more than you and my mustang is 14.2 hands. They are very strong sturdy animals and a lot of them have a nice size barrel to take up the leg.
As to height - the TIP trainer that worked Guinness has a 16 hand mustang and several in the past at her facility that were well over 15 hands. Taller ones are out there, you just have to look for them.