I was thinking about this a while ago, and I had posted something on CotI about this. Basically it drew from the Age of Sail classifications somewhat in the terms, but was applied by specific ship ship size and capability. I have a bit of a IMTU rule where most ships don't go above 100,000 tons, and the ship sizes reflect the military using the smaller end of the scale for economic reasons.
Here's a brief rundown:
100-200 tons: Gunboat/Escort
200-1000 tons: Escort/Corvette
1200-5000 tons: Frigate
5000-10000 tons: Light Cruiser
4000-2000 tons: Heavy Cruiser
12000-50000 tons: Destroyer (not because of the wet navy version but because it is the ship powerful enough to Destroy other vessels).