Ah, the eternal topic
I like to use max size (2,500 ton) engines for my warships, because they have the best fuel efficiency. Because of that class designs fall nicely into divisions based on number of engines. I sometimes further divide based on design purpose.
Frigate: 1 engine, 6,000-8,000 tons. Designed to provide support to other ships and light beam weapon capability, generally loaded with railgun or gauss PD. Railgun versions tend to be smaller to get better speed off one engine.
Destroyer: 1 engine, ~8,000 tons. Like frigates, except designed for anti-ship roles (either missiles or heavier beam weapons).
Escort Cruiser: 2 engines, ~16,000 tons. Point defense like frigates, but usually using AMMs since I find them hard to fit in to a smaller hull but wasted on a larger, multi-purpose vessel.
Light Cruiser: 2 engines, ~16,000 tons. Generally pure missile based anti-ship platforms.
Cruiser: 3 engines, ~24,000 tons. My smallest multipurpose combatant, usually armed with a combination of ASMs, beam weapons, and beam PD.
Heavy Cruiser: 4 engines, ~32,000 tons. Similar to cruisers, though I sometimes give them a heavy beam weapon focus.
Assault Cruiser: 4 engines, ~32,000 tons. The idea is to have a slow, heavily armored cruiser designed for jump point assaults, but it's mainly theoretical since I've never had an AI guard a jump point.
Battleship: 6 engines, ~48,000 tons. My command ships; about the same weapons as a heavy cruiser, with the extra space filled up with a flag bridge, extra armor/shields and PD, and a jump drive.
Battlecruiser: 6 engines, ~48,000 tons. Same size as a battleship, but gives up on being a command ship with extra defenses in favor of filling all that space with weapons.
I generally don't go bigger than that; the mechanics of shipyards and jump drives in Aurora means it always struck me as more efficient to have a squadron of battleships and battlecruisers instead of one or two dreadnoughts. If I ever get a game that far I might try going bigger, though. I also tend to eventually start phasing destroyers and frigates out, partly because fighters and battle riders start filling their roles and partly because giving orders to 30+ ships gets annoying.
Carriers I generally go with "the biggest I can make them" and then give it an appropriate name, I.E. light carrier, escort carrier, fleet carrier, etc.