I will let you know the ship scale I personally will be using, which is based on Volume.
Corvette: 0 m^3 --- 399.99 k m^3
Frigate: 400 k m^3 --- 2.99 mill m^3
Cruiser: 3 mill m^3 --- 19.99 mill m^3
Battleship/Battlecruiser: 20 mill m^3 --- 199.99 mill m^3
Dreadnought: 200 mill m^3 +
and then I have the special roles
Fighter: Low personnel craft, less than 200 blocks.
Carrier: 200 k m^3 +, Fighter Attack Role
Space Station: Any Size, but immobile or extremely slow
That's what I use anyways, and I have come across for my workshop creations, though, I've only made various types of fighters so far.