Levitating a rotating structure is not an insurmountable engineering challenge. Levitating such a structure with shifting loads within it is a different beast.
Still, if you could manage it, I'd say make it huge. The bigger it is, the slower you need to rotate it for a given g at the radius. The slower you need to rotate it, the easier it is to get in and out from the hub. Slow enough, and maybe you don't even need any fancy interface at the exit, just deal with the fact that the entry door moves and you can't take all day.
And also do this underground for radiation shielding.
According to Google, the largest known lunar lava tube is 360m in diameter. Put your centrifuge in there and you could get 0.5g at 1.24 RPM. That seems like something where you could get in and out of an airlock door at the hub without an epic leap and great timing.