Rendezvous with Rama · Arthur C. Clarke, 1973
How big is Rama in Rendezvous with Rama?
50 × 20km
A cylinder 50 km long and 20 km across. About as long as the Channel Tunnel.
From the bookReal world
Rama is a perfect cylinder, spinning once every four minutes. Inside it is hollow: 16 km across, so the land curves up on every side and there is ground overhead as well as below.
A 10 km sea rings the middle, with the island city of New York in it. The spin gives about 0.6 g on the plain and nothing at all on the axis.
These dimensions are the novel's own. What the book doesn't pin down is where each town sits, so their positions on the map are guesses, and a few sizes, like the Big Horn and New York, are rough.
See Rama in 3D