I usually have classifications that vary by purpose, with size only secondary. For instance, a frigate and a destroyer might be the same size, but a frigate will be specialized for PD/anti-fighter, whereas a destroyer is fast and armed with anti-ship weapons.
I'm the same way -- designations are more an indication of role rather than size, and I try to stick to the historical definitions:
Corvette: Small, fast, often single-weapon design. Roughly equivalent to the old "Gunboat" classification, pre-6.x
Monitor: Large, slow, lots of long-range weapons, no jumpdrive, low fuel/maintenance duration. Intended for in-system defense.
Destroyer: Small-medium, fast, sufficient weapons to outgun enemy ships of equal or smaller tonnage. Obviously this means destroyer designs vary based ono who they're meant to face off against. Anti-Swarm destroyers are
very fast (>10000 km/s) but a bit lighter on the guns. Anti-Precursor destroyers will be a bit larger, considerably slower, have built-in AMS, and more guns. Bear in mind that the name "destroyer" evolved from
torpedo boat destroyer, and that their role called for a ship fast enough and manueverable enough to intercept small but potent vessels which posed a threat to capital ships. In Aurora terms, any kind of defensive escort vessel should probably be a destroyer/destroyer escort.
Cruiser: Medium-large, decent speed, long fuel/crew/maintenance duration. Cruisers were originally meant to ply the shipping lanes looking for trouble, and/or be "the face of the Empire" in far-off stations. Initially was not a class of ship, but a role which certain classes could fill.
Frigate: Similar mission profile to cruisers, but smaller. Ideal for patrolling backwater systems or providing colonists with a reassuring military presence overhead.
Battleship: Large, heavily armed and armored, requires support vessels to remain "on station" for any length of time.
Battlecruiser: Essentially a fast battleship, trading armor/shields for better speed and longer duration.
Because I loathe the micromanagement of colliers, tankers, etc. needed with fleet support, most of my designs are frigates, cruisers and battlecruisers, although I will custom-build integrated fleets if I'm expecting a big dust-up with someone. And tend to rely on PDCs and monitors to provide in-system defense.