In Iain M. Banks, fairly rockin' culture novel, Consider Phlebas a good third of the book takes place on what is called the "Vavatch Orbital". An orbital is apparently any massive artificially created megastructure, similar to the Death Star or the titular Halo.
At one point in the book the Vavatch Orbital is described as a minor orbital, not as big as a ring or a sphere. Then later it is almost directly described as having a ring shape. Still earlier in the story it is described as a bucket being swung in circles, though this description maybe used only to describe how the orbital uses centrifugal force instead of actual gravity.
Whatever the size, the orbital has to have enough sea space for a Megaship (a city sized ocean ship) to travel for more than a century before reaching land again.
Wikipedia tends to suggest that all orbital's are rings, but then why the mention of a sphere, or that Vavatch is not big enough to be a ring?