If we are going to go by Starsector ship sizes, i made one a while back that is included in my mod.
These sizes are what starsector can handle regularly without consuming all memory and causing an OutOfMemory Crash, also the true and most commonly used sizes for sprite classification.
Arkship ~ 1750 - 2500 pix
Titan ~ 750 - 1750 pix
Dreadnought ~ 650 - 750 pix
Mothership/Supercarrier ~ 550 - 650 pix
Battleship/Battlecruiser ~ 350 - 550 pix
Superheavy Cruiser/Heavy Cruiser ~ 300 - 350 pix
Superheavy Destroyer/Cruiser ~ 225 - 300 pix
Heavy Destroyer/Destroyer ~ 175 - 225 pix
Heavy Frigate/Heavy Gunship ~ 125 - 175 pix
Frigate/Gunship/Heavy Corvette ~ 75 - 125 pix
Corvette ~ 50 - 75 pix
Heavy Fighter/Bomber ~ 35 - 50 pix
Fighter/Interceptor ~ 20 - 35 pix
Heavy Drone/Drone ~ 5 - 20 pix
/ValkyriaL
To be honest, it should state if the pixels are the length and/or width of the respective ship.
To have a really accurate scale of what the ship could be classified as, the actual pixelmass (total number of pixels the sprite has) would be of more significance IMO.
FRIGATES
Hyperion Frigate frigates HIGH 2418
Hound Frigate frigates LOW 2843
Wolf Frigate frigates HIGH 3032
Omen Frigate frigates HIGH 3037
Tempest Frigate frigates HIGH 3380
Lasher Frigate frigates LOW 3884
Vigilance Frigate frigates MID 3954
Brawler Gunship frigates MID 4402
Shade Phase Frigate frigates HIGH 4790
Afflictor Phase Frigate frigates HIGH 5707
DESTROYER
Buffalo Mk.II Destroyer destroyer LOW 9304
Hammerhead Destroyer destroyer MID 11496
Enforcer Destroyer destroyer LOW 12279
Sunder Destroyer destroyer MID 12594
Medusa Destroyer destroyer HIGH 12622
Condor Light Carrier destroyer LOW 13062
Mule Combat Freighter destroyer LOW 13590
Gemini Freighter destroyer MID 14405
Valkyrie Troop Transport destroyer MID 16474
CRUISER
Falcon Light Cruiser cruiser MID 18124
Eagle Cruiser cruiser MID 22093
Venture Cruiser cruiser LOW 22304
Aurora Cruiser cruiser LOW 23845
Apogee Cruiser cruiser HIGH 24082
Dominator Heavy Cruiser cruiser LOW 27120
Doom Phase Cruiser cruiser HIGH 29652
CAPITOL
Odyssey Battlecruiser capitol HIGH 40722
Conquest Battlecruiser capitol MID 51804
Onslaught Battleship capitol LOW 64812
Paragon Battleship capitol HIGH 73625