As there are multiple tasks to be fulfilled in space, there is no one ship size or function that can do everything. Through the centuries ship classifications have been standardized more for their role than size, but there is some common overlap.
Corvettes: 100-300m
These are the smallest warships in any given fleet. Most are not even FTL capable on their own, relying on larger ships to carry them between the stars. Much of their armament will be point defense or torpedoes, as their very quick nature allows them to get in close and launch a salvo, then bug out. This gives the humble corvette the ability to punch well above its weight class. Hangar space is limited, if non-existent on these types of ships.
Frigates: 200-500m
A frigate is more of a role, than a defined ship length. They are designed to be able to launch from a transport vessel and then fly in the atmosphere of a planet. The frigate would then provide ground support with its weapons and strike craft in its hangar during a planetary invasion. There are frigates that are designed for combat, sacrificing internal space for armor, shielding, and weapons, and those that are meant solely for ferrying vehicles and troops down onto the ground.
Destroyers: 300-1200m
The first true dueling warship and the most common. The larger ones have metaspace engines, allowing for independent maneuvers. They are the favorite of local planetary defense and pirates. More “all around” variants will carry a large complement of mag-cannons. However, as a destroyer is still fast, and far more maneuverable than its larger cousins, it can make great use of torpedoes and the shorter range corona-cannons, and pulse cannons. Armed with such weaponry, a competent captain can go toe to toe with cruisers and even do heavy damage to battleships. They act mainly to kill corvettes and fend off any encroaching destroyers, while harassing larger vessels.
Cruisers: 1200-3000m
The main frontline ship of any navy, they form the backbone of any strikeforce. Armed with a plethora of heavy mag-cannons and other assorted powerful weaponry, they can be designed to take on almost any foe. This classification is also where large hangar bays and docking stations for corvettes make their mark. The strikecraft complement and corvettes gives the cruiser almost unlimited flexibility. Those that are designed to fight within their own weight class will be skewed towards mag-weaponry. Meanwhile battleship hunters will sacrifice hangar space and point defense to add extra armor, shielding and a battleship level warden system. The biggest threat when facing a battleship is always their fusion cannon, and a standard warden system for a cruiser cannot stand up to such an armament.
Battlecruisers: 3000-4000m
This is a stopgap measure for a navy that wants a battleship, but does not have the fusion cannon for it. Heavily armored and armed, they can take on anything smaller than it, and can even brawl with battleships, as their larger size allows for a reactor capable of sustaining a warden system capable of shrugging off a fusion cannon hit. Though still, the lack of one leaves them at much of a disadvantage.
Transports: 2000m +
At 2km is when it becomes worth it to mount a metaspace engine on a transport vessel. However, most transport vessels easily fall above 4km in length. They are cavernous vessels, dedicated to holding thousands upon thousands of infantry, vehicles, and supplies in their holds. They are for all intents and purposes mobile cities. But such scale is necessary when sieging a planet, and fighting the possibly millions of defenders down below.
Battleships: 4000-6000m
Any ship large enough to hold a fusion cannon is deemed a battleship. This mighty weapon is the defining nature of the class. It gives the vessel the ability to smite almost any cruiser in one shot, long before it has the chance to fire back. And it is the only weapon capable of breaking a battleship’s own warden system. While a battleship will still carry plenty of other weapons, well capable of shredding almost any opponent, if a captain is not using the fusion cannon to its full potential, it is a wasted ship. The main role of a battleship is to destroy cruisers, and to face off against other battleships.
Dreadnoughts: 10000m +
This classification is for a ship that can mount multiple fusion cannons. Most dreadnoughts start at 12000m in length, with the largest ever recorded being 20 kilometers in length. They are a symbol of power just as much as a fleet killer. The Ostrus Kingdom and the Ashuan Republic, formely the Haskur Empire, both had around a hundred dreadnoughts at the start of the Gore War. Now they both barely have fourteen of these massive vessels. The multiple fusion cannons mounted on dreadnoughts allow them to effectively destroy almost any warship in a single salvo, warden system or not. It is ill-advised for a fleet to attack a dreadnought without one of your own, as the immense range of dreadnought’s primary weapons allow it to wreak considerable havoc before the enemy can even respond.