My TB is a compact 15.3hh who wears 6'3'' rugs unless they come up particularly big in which case we drop down to a 6'. He wears Horseware, Mark Todds etc and 6' 3'' is perfect.
Mine is just 16.2hh and he does have a few 6ft rugs which fit him nicely (I nearly bought a 15.2hh who i intially bought the rugs for - but it fell through)
I think alot of the rug thing depends on how slender they are too and some rugs fit horses better than others
My tb lady is 16h exactly and has 6ft rugs...what I have most difficulty with is that rugs fit her good around her chest without pulling on her and don't rub - she has very sensitive skin.
My mare is about 16h (haven't actually measured her!) TB. I find it depends on the make of the rug, I particularly like weatherbeetas on her and 6ft fits great, but horsewear and bucas she needs 6ft3.