I wrote a JTAS article not long ago about the Wyvern that had me doing a lot of similar research.
We know from the Drinaxian Companion's Rorix write-up that the empire preferred small ships for a variety of reasons. Note this line: "Having many captains of small ships meant that none commanded a vessel that could take on one of the few large warships the empire operated, making a rebellion by the fleet easy to crush providing the handful of heavy-ship captains remained loyal."
That says, first, that a lot of fleet doctrine was based around the emperor's paranoia and the despotic tone everything took in the latter years. The conclusion I came to for my article was that the sindal fleets were tightly regulated in terms of size and scale. The emperors wouldn't want any duke to get so powerful that they could rebel without getting taken down by all the others, so each fleet had a very limited number of ships at any given size category.
It also says they only had a 'handful' of heavy ships. My headcanon is that the 50,000dton warship currently scrapped out by Thebus was the emperor's own flagship. I figure each duke probably had a ducal capital ship in the 20,000 range, and then maybe 4-6, 'major' battle ships at the 10,000t level. Most of the real work was being done by sub-1,000t, or more likely sub-500 vessels, and that these small ships were more severely restricted operationally than you'd see in the third imperium. The Rorix only has a jump 2, for example, so I assume that was fairly standard for the smaller craft.
Of course, theses grander ships would be much major prestige postings and would need to be designed accordingly. I figure they'd have lavish accommodations more suited to luxury yachts then battle craft and decorated accordingly, and gosh, can't you just picture each duke trying to outdo the others for who has the nicest ships? Just imagine: ships that make the harrier look pedestrian! I stated out a few of them for my own campaign, it was a lot of fun.
I'm looking forward to this adventure you're writing, Morgoth!