Just a little gravity, combined with proper exercise and diet, may be all that is required to keep humans healthy on long missions. I'm sure this is being thoroughly researched by the space powers even as we speak, as we do our planning for Mars.

Also, the amount of time spent at different g levels may be critical. Perhaps a few hours a day at full g are all that's needed to keep us healthy, and I'm sure sleeping quarters and work spaces will be in the high-g areas, where people will spend most of their time. Cargo storage, air locks, machinery spaces and docking facilities will probably be in the low-g areas.

Remember, gravitational acceleration increases with the radius, but with the square of the rotational velocity, so you're better off with a big station spinning slowly than a small one spinning rapidly. The big problem will be if the spinning station becomes unbalanced and starts violently precessing, say as a result of even a minor collision or internal explosion or other unanticipated mass transfer. A large, lightweight (flimsy ) structure can probably be designed to handle a constant spin, but any oscillation would probably shake it apart.

(Lots of good plot ideas there, like sabotage of the automatic mass distribution and balancing system!)