I suppose that depends on the way the designer uses the interior volume. Not all same length hulls have the same interior volume.
I've seen 42 foot interiors which are no bigger than 35's. And this all depends if you live aboard or weekend. I would like a stand up aft cabin for no other reason that in our aft cabin double one person has to move for the other to get out of the bed. And perhaps a second head because of the privacy IF the V is occupied. We don't usually have anyone sleeping forward to walking 12 feet to a head is certainly not an issue.
I think around 40 with the increase in beam and volume would be optimal for a couple living aboard... and still keep the forces controllable.