They can be made to have almost any size. Even with a biosphere. Like the rings in Halo. You can walk and enjoy air that is as fresh as in any natural environme

They can be made to have almost any size. Even with a biosphere. Like the rings in Halo. You can walk and enjoy air that is as fresh as in any natural environment. The real disadvantage is that it is more fragile than a planetary environment. Also, civilizations living there would have to produce technicians that can maintain its systems. Planetary civilizations could choose to become low-tech.