6'4" in an Allmad 35
I lived aboard my Allmand 35 for 4 1/2 years. Most of the time was spent in the Caribbean.
The only place that I could stand straight was under one of the 3 hatches. Most of the rest of the boat had 6'2" of headroom. It wasn't an extreme problem since in a real cruising situation, you are either off the boat, (visiting others or ashore), on deck, or sitting down below decks. I'm not going to tell you that after 4 years I didn't wish for more headroom, but if I had to do it over again, the headroom wouldn't be a stopper. I would rather have 2" less headroom then 2" of additional windage above decks.
I also spent 6 months living aboard in Lake Ontario, and a winter in the Chesapeake.