The Galactic Navy has five designations for ship classes, with each only being able to effect those one size class lower and higher in any effective means.
Personnel Vehicles - Smallest of the usable vehicles, normally one person piloted/driven holding a number of people. Cars, Tanks, Plains, Trains, Troop Transport, armored or unarmored this is the normal set of vehicles that we are used to, most anything conventional
Light Vessel – This is the intermediate between space fairing and planetary vehicles. These include space to planet transports, fighter space ships, and all others that are designed for moving between the atmosphere and the stars. This would also include something like conventional aircraft carriers
Frigate Ships – This is similar to the frigate class in the navy but on a planetary scale, these are the most seen ships designed for any faction in the government, private, and commercial. Under United Earth Council law these are the largest ships that can be privately owned. Gun Ships, Freighters, and Transport ships are in this category, normally carryings between 20-50 people for running the vessel.
Carrier - Made for housing and repairing between 2-5 Frigate sized ships along with a decent quantity of Light Vehicles. Normally too costly to field many in any fleet they are kept from the battlefield as operational command these ships house facilities far above that of the standard sweets needed to protect, allowing for recreation, science, and even manufacturing. Besides Galactic Navy there are only a few in use, a few for manufacturing controlled by some of the larger companies, and the majority of the privately owned ones are resort locales for exotic vacations away from any local authorities. Houses around 500-1000 staff to run the facilities with the ability to hold hundreds more if the need arises.
Titan Class – These are largest ships known to humans with only 3 in existence; Kronos, Hyperion, and Theia. Each houses about 2000 people running the ship, along with commanders, can be up to 10000 soldiers, multiple facilities, and full dry docks for repairing all other smaller ships.
Ship Combat: +2 Gunnery and -1 Pilot Vessel on maneuvering against ships one smaller , -2 Gunnery and +1 Pilot Vessel on maneuvering against ships one size larger.