Rendezvous with Rama · Arthur C. Clarke, 1973
How wide is the Cylindrical Sea in Rama?
10km
A 10 km band of water running all the way round the inside of Rama.
From the bookReal world
The plain stops at a 50 m cliff over the sea. On the far side, past the towers of New York, is a southern cliff about ten times as high, with no way up.
When the crew arrive the sea is frozen. As Rama warms on its way towards the Sun, the ice breaks up, and Ruby Barnes skippers a boat built from spare parts across it. The water is a poisonous organic soup.
The 10 km width is from the book. New York's size, about 10 × 3 km, is rough.
Stand at the edge of the sea