The diameter containing a fraction of the current is used as the measure of the charged particle probe size. This has, particularly in the case of analytical microscopies, proven to be a more practical measure than those based on widths of the current density distribution. The diameter of current carrying beams of finite brightness and energy spread is enlarged by spherical and chromatic aberrations of the lens sytem and (for electron probes) by diffraction. The only correct way to add these contributions to find the total probe size is by elaborate exact ray-tracing or wave optical simulations. We have constructed a simple root-power-sum algorithm, in terms of analytical functions, for the approximate addition of the various individual contributions. The total probe diameter yielded is quite close to that given by exact calculations.