Horseware do recommend that you go one size up for their turnout rugs. One of mine is in a 6ft 9 Rambo but in another brand he's 6ft 6 (which is an indoor under rug.) Not sure if that's any help!!
Agree with above, my mare takes a 6'3 weatherbeeta but her 6'3 horseware rugs are a little on the tight side, I wish I'd have known when ordering as I'd have gone a size up!! She'd be better off in a 6'6.