My definition of boat size would be based upon the ease of maneuverability in a marina. If you can pull up to a dock with the ease of anything between a rowboat and a center-console fishing boat, fend off and tie it up solo, it's a small boat. Anything else requires an adjustment to the type of skills and techniques to get her from point A to point B. In some ways, it's the sailor that defines a small or large vessel.
"It's not the size of the boat that matters, but the sailor at the helm."
-Will (Dragonfly)