The largest starship I'll probably ever design, is a work-in-progress, since I've got tons of mapping yet to do - because its so massive. It already has some cool details, and some of it is designed, so worthy of posting, I think.
This is the Argos, a Generational Colony Base Ship of Super Colossal scale, using the Starship Operations Manual rules this has a crew size of 100,000 and as colony base ship has 5 times that as colonists, so 600,000 complement total. So this is a damn big ship. It measures 396 miles long from stem to stern. The fore portion contains the bridge and operation bays, while the center is massive bays containing ore processing, foundries, manufacturing centers, vast warehousing, hydroponics/food processing, pharma labs, science labs, full maintenance facilities, massive hangers for tiny to gargantuan ships docked inside - the industry workhorse required to establish permanent colonies. A mass transit tram moves 2 ways down the center at high speed taking colonists and crew from the torus to the bridge or engineering, though it still takes an hour transit between. The aft portion of the ship is the massive drives and engineering taking it across vast distances of space - the kind of ships used to establish the colonies of the Colonial Space, of my Kronusverse setting. Someone helped calculate a very rough estimated weight of this thing at 500 trillion displacement tons, at 13.5 cubic meters per ton.
The torus section alone, I did the math, is 9.96 miles wide, 192 miles in diameter, and 603 miles in circumference, which is 5,991 square miles, at 90 occupants per square mile being the ideal non-urban population density, the torus holds a population just over 600,000 residents. I figure six cities of 50,000 evenly spaced around the circumference, with smaller communities of 5000 - 10,000 interspersed between them, with plenty of farmland, hills, forests, lakes and rivers. Using light effects the sky is blue, real clouds circulate overhead that can release rain, and an artificial sun rotating slower than the torus rotation to simulate the passing of a day. A high speed, mass transit tram runs the circumference of the torus inside the walls flanking the torus interior.
Since the torus is 96 miles in radius, in order to achieve 1 G of centrifugal force, it spins at only .076 rotations per minute (roughly 13.5 minutes per revolution), but that's still spinning at 2,753 miles per hour at the outer edge of that ring! A weighted counter-balancing ring spins in the opposite direction for stability.
The illustrations below shows the exterior view of the Argos. A 3D illustration view of the torus interior. Then a map of sliced sections of the entire torus interior area.