The lowest class of ships are mostly relics of previous times before major artificial intelligences were commonplace. These ships vary in size, but none of them reach a kilometer long because every part of the ship is controlled, for the most part, by one of the intelligent species. Some of these ships have rudimentary artificial intelligence in the form of targeting systems and a precursor to true auto-pilot, but almost all decisions are made by the intelligent lifeforms that inhabit the ships, whether human or otherwise.
All ships containing an artificial Mind have a class of intelligence and a size class. Generally, the size of the ship corresponds directly to its intelligence, but this is not necessarily the case. While the intelligence of a ship is quantifiable, this is not to say that some ships have authority or contempt over each other due to intelligence. The only ships with authority are the small number that are elected to the Administration. The Minds of the ships start with the sum of the Collective's knowledge and form their own personalities over time. It is not analogous to the birth of a biological species because they are "born" knowing everything, but their development over time is not dissimilar as it is shaped by their experiences through the galaxy.
The smallest class of ships is the probe class. These ships can have Act 1 or 2 Minds, but not 3 because of their limited size. The average size of the probe class is around a kilometer long with large variances in width. This level of ship has no space for biological passengers unless specifically necessary so as to not limit its ability to accelerate. These ships usually carry the bare minimum of weaponry required for the solar system in which they are assigned, but carry huge amounts of sensor arrays and measurement devices. They use this vast array of equipment to collect information about their solar system and transit their findings to the rest of the Collective ships. (Memory is shared among all Collective ships, but the smaller ships have to delete extraneous data whereas worldships have copies of everything the Collective has experienced. Smaller ships missing any piece of data can ping worldships for it and have immediate access to it. This is effective for both redundancy in the case of a rogue ship and so that all Collective ships have access to all data in order to make the most informed decisions.) The primary objective of the probe class is to collect information, so if there is ever the spark of a peacekeeping issue they usually send for a higher class of ship.
The cavalry class consists almost entirely of Act 2 Minds, but on rare occasion one is made large enough to house an Act 3 Mind. Calvaries are made from 10-50 km long. The cavalry class is large enough to accommodate up to 1000 passengers, depending on the need, and as such cannot accelerate to the degrees that probes can. The cavalry class is built for interplanetary combat and has a lower percentage of sensor/total mass than the probes do. Its sensors are still expansive, just backed up by significantly more firepower. The cavalry class move between solar systems more than the probes do, but the closest probe emergency requires them to redirect to that solar system. The primary objective of the calvary is to help the probes, but for the most part they are also involved in data collection in whatever solar system they happen to be in.
Commonly referred to as worldships, the planet class all have Act 3 Minds. These ships are often accompanied by a cavalry and multiple probes, and mostly server as massive inter-solar-system transport. They are too large to move within the strong gravitational pull of most suns, and so the cavalry and probes act as transport in and out of the solar system to the worldship. These behemoths are few and far between, but the ones in existence are 1000-1500 km long, able to support populations up to 2 million humans and various amounts of other species, depending on their sizes. Some communities that live within the worldship choose to never leave and sometimes individuals spend their entire lives on board. The environment inside each individual world ship are entirely controlled by the ship's mind and vary according to its specific peculiarities. An account from the perspective of a traveller on a worldship can be found here. The Mind of worldships can portion parts of itself for subroutines that control the different parts of the ship, as can all Minds, but due to the size of the worldship they are more likely to develop a mental illness due to the fragmenting. The primary purpose of a planet class is to transport and provide for its passengers, but that does not preclude it from carrying a suite of weaponry and sensor arrays. Worldships basically do what they want, but this often aligns it with the interests and needs of the other classes of ships.