The length limit historically is probably held by Alain Colas, on Club Med, in the Observer Single handed Transatlantic Race of 1976
As for displacement, ditto -- Club Med's keel alone weighed 85 tonnes (depleted uranium, courtesy of the French government, so about half the size of a lead keel) ... that's 190,000 lbs hanging under the hull
However the sheer macho record surely goes to Eric Tabarly, in the same race, on Pen Duick VI.
This was possibly the most heavily equipped ocean racing maxi ever built, and was intended for a crew of dozens of strong guys, whereas Club Med was designed specifically for single handing -- although not well enough, as it turned out: the halyards chafed through, one by one, in the race in question -- which incidentally Eric Tabarly won, after his self-steering broke (AFAIK
the ONLY concession to being solo which was fitted to the boat), and he hand steered 24/7 for about the last two days..... He even put up a friggin spinnaker, and this was before the days of virtually all labour saving devices for solo sailors. Certainly NO roller reefing or furling. Headsail sheets were wire rope, the kind you tow trucks out of ditches with; shackles were D shackles the size of a small fist.
Colas, OTOH, couldn't even finish the race: he ended up pulling into Newfoundland IIRC because one by one all the sails had fallen down, and it was not possible for a single guy to rehoist even one of them, let alone eight. (He had brought a whole team on board specifically to raise sail, and they all had to get off before the warning gun)
Which only goes to reinforce the excellent point made by Foolish Muse about a page back: they both had stupidly inappropriate boats, but one guy coped when things went wrong, and the other couldn't.
Tabarly was deservedly a sailing legend, absolutely one of a kind, but single-handing was really his forte: he had none of the leadership skills it takes to run a big boat successfully other than the ability to do most anything himself better than 99.9 of potential crew. A generation of starry-eyed, hero-worshipping young French sailors were (generally) privately disappointed when they finally made it onto his crew list and found out his limitations as a manager of men.