My mare is 16.1hh 5 year old (hopefully may still grow another inch !) warmblood (Holstein x TB) She is in 6 ft 6 rugs, although 6ft 3" fit in length they are a little too tight across the chest.
well my old 16hh TB mare was in 6' (although did have a couple of 5'9" rugs that fit) i'm now using the exact same rugs for my current horse who is somewhere between 15.2 & 15.3hh CobX (i also managed to squeeze a 16.3hh TB loan i had into a couple of the rugs last year - although they we're a little like miniskirts!)
Think my cob is about 14.2h (though really should measure him!) and had to get 6' 6" rugs!! Tried 6' and only just big enough for him (backseam wise, and 6' 3" was same backseam length), so being 2 thought I'd get a bit bigger so it also fits later down the line.
Depends on the rugs. My horse is a 15.3 ID and has 6'6 turn out rugs 'cause he lives out 24/7 and I like them to cover his bum well. His under rug is 6'3 as I want it to be completely covered by the top rug. Most of my stable rugs are 6' & 6'3 and sweat/travel/day rugs are 6' as I don't like these to hang over his bum.
Also, different makes of rugs vary, some are more generous in fit than others.
17.1hh dutch warmblood in 6ft 6-6ft 9" rugs. Depends on length of back and build of horse i.e if heavy on shoulders like a cob then possibly a larger size rug than a TB of the same height.