  We demonstrate a new 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 discuss how micron-sized
beads can be used as probes for optical aberrations introduced by the
experimental setup.
