- We demonstrate a method for determining the radius of micron-sized particles trapped by a vortex laser beam. The technique is based on measuring the rotation experienced by the center of mass of a microsphere that is laterally displaced by a Stokes drag force to an off-axis equilibrium position. The rotation results from an optical torque pointing along the direction opposite to the vortex beam angular momentum. We fit the rotation angle data for different Laguerre-Gaussian modes taking the radius as a fitting parameter in the Mie-Debye theory of optical tweezers. We also demonstrate how the setup described here can be employed to characterize the astigmatism and spherical aberration introduced by the experimental setup, while at the same time determining the height of a microsphere with respect to the bottom of the solution within which it is contained.