I've been working on a recreation of Rama, of which a shell has already been posted on the dock, and I can confirm from many hours of searching that it is the largest ship up for download on the dock (2.7km). When I finally get it done it'll be the largest completed ship on the dock, beating out the Omega Ragnarov, the next in line. I'm also going for largest station on the dock (6km).
More along the lines of a curiosity and an experiment towards discovering the limits of StarMade, I once build a ship that was 21.7km in length, 1m in width and height. Aside from some minor rendering issues with slabs, wedges, and transparent blocks, it didn't lag very much. Since it was a completely useless experiment, I won't be releasing it. But from this test I can guess that lag has more to do with block count than dimensions. Another interesting experiment that I undertook was when I tried (and failed) to build a 1:1 Death Star 1, revealing that there is a limit to dimensions, and that they lie around the 30km mark.
As a final note, I'd say that for ships, 3 km is probably the largest a reasonable ship can go. Any larger and it will probably require more than the average system components to be able to be played with *cough* 32 GB of Ram *cough*. As far as stations, I'd say around 10 km, but my knowledge of the limits of stations is extremely limited unfortunately. Any additional information I can gather I can post here.
