[member="Kara Vi'dreya"] I know exactly how big two kilometer ships are. I also know that so far as navies are concerned , those ships are basically the standard frontline ships(considering that many 2km ships are mass-produced holdovers from the pre-2km-are-minor-only days. We've got a perfectly plausible rule set for ships 3km long, restricting them to a production level of "limited" or "semi-unique."
I'd hardly consider the first Death Star to be a perfect example of why it wouldn't work - the 120km battle station was completed approximately twenty years after construction first began, a ratio of 6km every year. Applying that to 3km long warships requiring a lot less overall work(since the 3km section in the Death Star example is a whole 'slice' of it, and thus would require more work than the comparable warship), a minimum of 2 3km warships could be constructed, fitting ships into the category of "limited" or "semi-unique." Would the production values of these ships be high? Of course, since we're talking about so few vessels per class, but there is no reason a major faction cannot plausibly construct such ships.
Hell, if you really wanted to, a simple reorganization of the starship nomenclature could allow for more proper ship limitations, with 3km as the maximum limit. But it's k - I'm me, so I guess my idea has no merit. Will wait for [member="Ashin Varanin"] to bring it up sometime.