Always depends on the weather. Not only the length but how the boat is built is important. I Would say that a general rule of thumb, would be something around a 21-24 Ft with good freeboard at the smallest. I'm sure someone has been out there in a little runabout in perfect conditions some time, but not me, like never, ever!
I would never be out there, except in great weather anyway!
It is a beautiful place to go in the right conditions.