Maybe a bit late, but I'll add it anyway
These are essentially the designations I'm using in my upcoming Starmada universe setting. They are not set-in-stone and revolve more around the role of the ship rather than its physical size.
By order in the fleet list:
Gunboat - A lightly armed police ship intended for nothing more than local system patrol.
Corvette - Fast, inexpensive and lightly armed patrol ship. Designed for pirate interdiction (pirates play heavily in my setting) and convoy escort. Should operate in packs. Usually employed by militias or corporations.
Frigate - Generally the smallest Navy ships. Used for convoy escort, patrol, and system interdiction. Hyperspace coils are too small to use for effective scouting.
Destroyer - Smallest ship used in fleet actions. Destroyers in my universe are usually specialized to fill specific roles, such as escort, skirmishing, or scouting. Also generally the smallest ship capable of operating independently.
Light Cruiser - Most common Navy warship. Offers the best compromise between cost-to-operate, weapons, defenses, and speed. Various models exist to fill different roles.
Cruiser - Standard Navy warship. A Navy patrol group is usually made up of a cruiser with several light cruisers and various amounts of smaller escort ships. In battle a cruiser is suppose to dish out and absorb damage, and specifically keep the attention of the enemy cruisers off the light cruisers (while they chew apart the hostile forces). Cruiser are generally the largest Navy ships used outside of wartime.
Battlecruiser/Heavy Cruiser - Essentially an upgunned cruiser, usually with more protection and armament. Somewhat more rare because of the cost in both material and logistics. Not designed to fill a particular role, more to counter the increasing number of cruisers. Not generally deployed outside of wartime.
Grand Cruiser/Stardreadnaught - Experimental starships designed to bring overwhelming, decisive firepower to a battle. About 50% larger than a cruiser. At the current point in my timeline none have been deployed in active duty, though several prototypes exist. Warships of this size have not been built due mostly to the cost and logistical requirements.
Raider - Small, fast starship used mostly by pirates. Heavily armed for their size. Usually very weak hulls as their maintenance and build quality is highly suspect.
Warcruiser - General designation given to any ship with firepower comparable to a Navy fleet warship.
Escort - General designation given to any ship designed primarily for convoy protection.
System Monitor - Usually a small ship (can be purpose built but often a conversion) that is slow but very heavily armed. Designed to defend static locations like space stations. Doesn't often have a hyperdrive to cut down on used mass. In battle a system monitor is considered "successful" if it manages to get its weapons fire off before being blown to bits.
That's basically where most of my designations fit. They're based more around role rather than size, so it's possible to have a "destroyer" that's equivalent to light cruiser tonnage, for example.
(if anyone is wondering about size, my cruisers sit at 16-18 hull, 7-8 armour, and a speed between 3-5)