Reading about the problems of tall people make me happy that I decided in my teen years to stop growing before reaching 6 ft. I wouldn't base my two week trip around clothes shopping. If you are tall, not only look at the head but also look at the feet. No other issues really. I guess if I were more petite in frame, I would probably have managed fine.
Note that this thread has not been updated in a long time, and its content might not be up-to-date anymore.
Tall Blond Woman Standing in a Doorway in a Black Dress
Slippers have never been an issue - it's not like you'll be wearing them a lot anyway - I wouldn't bother taking your own - they'll have slippers that fit. Shinkansen have tons of leg room, even in ordinary class or non-reserved sections. As for men, the yukata may be a bit short for him. Dear visitor, if you know the answer to this question, please post it. Looking at all the accomm photos for about a billion ryokans and hotels ok that's an exaggeration but I looked at so many when deciding then booking out accomm I didn't see any beds with footboards, so I don't think the height will matter for sleeping regardless of whether you're in Western or Japanese style rooms.