a nice article on the question. As you see, you can get earth gravity with a pretty small wheel if you spin it fast enough. But this creates lots of problems. For one thing, it feels really weird if gravity on your feet is greater than gravity on your head, and nobody knows if people can adapt to that. And this:
The Coriolis effect gives an apparent force that acts on objects that move relative to a rotating reference frame. This apparent force acts at right angles to the motion and the rotation axis and tends to curve the motion in the opposite sense to the habitat's spin. If an astronaut inside a rotating artificial gravity environment moves towards or away from the axis of rotation, he or she will feel a force pushing him or her towards or away from the direction of spin. These forces act on the inner ear and can cause dizziness, nausea and disorientation. Lengthening the period of rotation (slower spin rate) reduces the Coriolis force and its effects. It is generally believed that at 2 rpm or less, no adverse effects from the Coriolis forces will occur, although humans have been shown to adapt to rates as high as 23 rpm. It is not yet known if very long exposures to high levels of Coriolis forces can increase the likelihood of becoming accustomed. The nausea-inducing effects of Coriolis forces can also be mitigated by restraining movement of the head.If you take 2 rpm as a safe figure, you see that you need a rotating wheel with a radius of 224m (735 feet). Now maybe you don't need full earth gravity; maybe half a g would be enough to prevent health problems (nobody knows). That could be done with a radius of 100 m (330 feet). Either way you need a very big space ship.
So to use this strategy on a 21st-century Mars mission you would probably need a rapidly rotating wheel. Perhaps the astronauts would spend only part of the time in the rotating part of the ship. Again, nobody knows what sort of regimen of part-time gravity would mitigate the harmful health effects of extended zero gravity.