Basically the newest ship is the most powerful. In Consider Phlebas the Killer-class ships Trade Surplus and Revisionist are typed as Rapid Offensive Units (used for front-line units), but when we find a Killer-class ship later in the series, the Attitude Adjuster in Excession, it's been demoted to a Limited Offensive Unit.
At the same time, in Excession, the Gangster-class Heavy Messing is now what is consider a ROU; from a bit of description of the baroqued ROUs stored in Pittance we find an approximate size:
The sixty-four ships stored in this hall were Gangster Class Rapid Offensive Units; slim segmented cylinders over two hundred metres tall and fifty in diameter.
(Note that the Affronter heavy cruisers are much larger; a couple of kilometres long, but they are at a comparatively lower tech level, and also have crews.)
But based on the AI-commanded warships built by Sleeper Service, even the Gangsters are being replaced (note that the highest-quality ships are built in the fewest numbers):
- Type One Offensive Units (roughly equivalent to Abominator class prototype): 512.
- Type Two Offensive Units (equivalent to Torturer class): 2048.
- Type Three Offensive Units (equivalent to Inquisitor class prototype, upgraded): 2048.
- Type Four Offensive Units (roughly equivalent to velocity-improved Killer class): 12 288.
- Type Five Offensive Units (based on Thug class upgrade design study): 24 576.
- Type Six Offensive Units (based on militarised Scree class LCU, various types): 49 152.
At this point the Killers are now barely better than a Limited Contact Unit.
Then we meet a full Abominator, the Falling Outside the Normal Moral Constraints in Surface Detail where the single ship takes out an entire squadron of GFCF ships in a fraction of a second. A squadron that would be capable of a one-shot kill of a Torturer class. (In Surface Detail we also see that the Psychopath class has been downgraded to Fast Picket from Very Fast Picket in Excession.)
It's also worth noting that Culture warships have a recognizable size/profile:
The three craft were unambiguously warships; slightly longer and fatter than the Grey Area itself but tapering at either end to points surmounted with large spheres. Spheres which could logically only contain weaponry. Quite a lot of weaponry, judging by the size of the globes.
(The quote refers to 3 of the T3OUs fabricated by Sleeper Service.)
So it's safe to say that Culture warships are approximately cylindrical, two to three hundred metres long and about a third to a fifth as wide. The specific shape varies (IIRC Torturer class were a single tapered section at the end of a longer cylinder) but they are all basically the same size. The difference between classes is that the newest class is the most powerful.