6"3/6"0 should be okay depending on the build
I have 6" on my 15.3hh mare 🙂
My 4 year old is 16hh but quite petite since he has not much muscle and he’s 75”
You measure your horse from the chest to the back side
My 14.2 is a 5"9 so I'd say 6"0 or if he's broad chested or heavy built I'd go next size up so that the rug would not rub his mane or so he will be comfortable
if you say at the store if you are going to the store if you ask that you have no idea what size she is they will help you with that by asking how big they are
Measure from the middle of her chest to her point of buttock. Then google rug sizes and it'll tell you what would fit. Usually for 15.3hh I'd get a 6'