It's not the height that's relevant but the length! Also the type of rug as some are generously cut and some aren't. If the horse is chunky it may need a bigger size depending on how the neck is cut.
My lad is as wide in the chest as he is along his back, hence mark todd rugs fit him sooo well, he needs the 6ft 9 as he uses it up in his width - if that makes sense
When I first bought her she measured up as needing a 6ft, which I splashed out on buying her a lush wardrobe off, and then she outgrew them
Then I measured her with topline, and in work, and bought her 6'6" rugs... Now she's out of work though, those are too big so for the third winter in a row I've had to kit her out in a new size!