Dunno
About a month ago, I was curious and looked it up. The best explanation I found said 33 feet or under, and I was suprised it was so big. Now I can't find my source.
Googling today, I saw a reference to 65 feet which I think is nuts.
I think NOAA leaves it undefined for a reason. There are some tiny craft I would trust in heavy seas, and many large ones that I would fear on a flat lake.
I'm amazed by some of the top-heavy stink pots in the harbor. Recently, I was talking to the owner of a 65-foot boat that required more than a month to deliver from Seattle to Southern California last summer. And, I can see why --in anything over 10 knots, or four-foot seas on the beam, I would fear it capsizing. In that, I would heed the "small craft" advisories.