As you already know there is no agreed scheme of nomenclature and some of it is role based, some of it tradition.
But in my little world:
Corvette - less than 150ft and 1000 tons.
Frigate 200-350FT and less than 4000 tons.
Destroyer - 300-400ft and less than 8000 tons.
Cruiser - 400-500ft and less than 15000 tons. (Obviously this characterizes all the new Aegis "Destroyers" in service today and they should rightly be called Cruisers at 10000 tons)
Boats - All submarines of any displacement and day craft/landing craft/river and littoral patrol. PLUS anything the Coast Guard sails:).
After 15000tons it's all role-based. Carrier or defunct Battleship.
But could you imagine a 45T, 700ft "Balttleship" with all that steel armor...AND 396 VLS tubes?
Thanks, that makes sense to me.