Deriving the answer to this question can get involved. I am giving the formula only and am assuming you are looking for the magnetic field B at the center point of the sphere. If the sphere rotates with angular velocity along, say, z axis, then the formula for B is as follows:
[tex]\vec B_0=\frac{2}{3}\mu r q \vec \omega[/tex]
(with mu the magnetic permeability)
