Think it was Hugo Vihlen with a 5ft 4in boat "Father's Day" in 1993.
The 'smallest boat to cross the Atlantic' is one of those challenges that has produced a number of curiousities designed purely for the stunt, which can hardly to be called boats. I recall one that was deeper than it was long. Of the shortest 'real' boats to cross, I would rate Robert Manry's Tinkerbelle (13'6") about 40 years ago as among the most interesting.
What about smallest to South Pole - Kotare, Kingfisher 20 (20 feet). OK, not that small, but lordy, must've been nippy amongst them icebergs. Good for the GnTs, tho'...
("Sea Egg"). Does anyone remember this? John Riding's 12 foot yacht in which he attempted to circumnavigate back in the 1970's. He crossed the Atlantic and the Pacific, riding out one hurricane which I believe was written up in YM, but he vanished without trace in the Tasman Sea. He's almost completely forgotten now, but was a great inspiration to shoe-string voyagers of the time, including Shane Acton.
The smallest boat to have attempted it was the 3'11" Vera Hugh II belonging to Hugh McNally. The website (indicated elsewhere) shows that he got to Gran Canaria and was going to attempt the Pond last year. .. ?? He already crossed in the 5'4" Vera Hugh.
This list does not mention Laurent Bourgnon's 1986 crossing in/on a Hobie Cat.
Tinkerbelle was a really pretty boat and quite up to the job. I remember well Robert Manry's arrival. He was feted in a big way then went home to the US where he was run over and killed.
I met a sweedish guy in Horta this year that came over from Bermuda in a 14'er. bit of a nutter if you ask me but quite likable. It took him 4 weeks, I gather he then sailed north and left the boat (you can just call it that) in the scillies.