What some people seem to be missing is that military ship classifications weren't originally based on mass or size of the ship, but its role in the fleet. That role is what dictated the ship's speed, armament, defensive capability, and all that together dictated the size of the ship. As technology allowed faster ships and heavier armor, the same ship classification could include a larger ship. The role of a destroyer required a smaller more nimble ship, a battleship needed to be big and armored, with big guns. Sounds like Panpiper and I may be of the same generation, the system he laid out pretty much matches the one I've always used, and for about as long. I still even have my original books of Mercenary, High Guard, Scouts and Merchant Prince. :D
Civilian ships have never really ever been classified by mass or size, except as a secondary characteristic. Usually they're classified by their task: Fishing Vessel, Tanker, Container Ship, Tug, Liner etc.