Mustangs do tend to run small, but there are some larger ones to! One we had looked like she had some draft in her and was probably 15.1 or.2, she was a big girl. She would have been great for you!
I also knew of one that was 15.3 as a yearling (born in a holding facility)! So if your heart is set on a mustang you could probably find one to suit you. (I'd look more at build then height)
If it was me and your not stuck on one breed, then I wouldn't worry about the breed, just find a nice, stout horse, that's good w/ cattle. Generally you won't go wrong w/ a nice quarter horse, Paint, or Appy (although my Paint is afraid of cows).