I attempted to build a ship that was I want to say 3km in length 800mx600m, it was prior to update 1.4? I want to say, and the server I was on, even with a dedicated server it couldn't really handle it. One of the moderators who I befriended prior to that was like, man stop please, the server cannot take it. So it was removed unceremoniously. I have like one photo of the front of it when I was just making the shell. I had started making it with black hull, kinda hard to see, and the idea was that if they ever allowed for ramming collisions it would do a unsc infinity. I also tried to make a home base the size of a sector, I attached that as well, didn't turn out too well either, I wish they would solve that chunk loading issue. Looking back I must of lost many of my old screenshots, I had uploaded a lot to photo bucket, but lost them all when they went pay to use.
As for ships that are large and work, maximum safe and usable size Is anywhere within 500m-500m-500m, you can get away with one extreme to like 800m in a direction but its not really recommended. The game, and servers just really cannot handle it. I know someone will attempt to prove me wrong because there always has to be that guy, but realistically, what I quoted is "pretty close" to accurate.