Meteorain said:
They usually serve as Flagships, but it isn't as uncommon as you indicate. They also share that role with Emperor, Victory, Apocalypse and Oberon Class ships.
Cruisers are approximately 3km in length. Still twice the length of an ISD.
Seeing as the numbers of fleet strength is given in various gothic texts, yes the numbers are fairly accurate. The Apocalypse class itself is incredibly rare and the Imperium has lost knowledge of how to even make anymore of it, so they are careful with their use, the IoM only has a handful of them left after the Gothic war. The Oberon is also considered a very rare ship as well.
The Emperor and Retribution classes are the more common battleships, and even then their presence is generally as fleet flagships. Imperial Grand Cruisers are quite large and almost Battleships, and more common.
Suairyu said:
Naval warfare is pretty brutal in its destruction of the myth that size means anything. In straight firepower, defence and non-FTL manoeuvrability, how do they compare?
Sublight speeds for 40k ships have never really been well documented with wildly different takes in various sources. They are sometimes presented in battle as very slow lumbering behemoths, while other times they are zipping around each other as if dog fighting.
Defense wise it is hard to say since you have made up armors and alloys in each universe and how they react to each others weapons is almost impossible to stay. Many will say that size means nothing, but this is not really true as generally you will have far more of everything on the ship, more redundancies, thicker hulls, more armor, etc.