The class system I use for naming combat ships is as follows:
Mass - Class
< 100 - Fighter
< 250 - Gunship
< 500 - Bomber
< 2,000 - Light Corvette
< 8,000 - Heavy Corvette
< 20,000 - Destroyer
< 55,000 - Frigate
< 80,000 - Cruiser
< 200,000 - Battlecruiser
< 500,000 - Battleship
<...