When fluorescence correlation spectroscopy (FCS) in combination with a confocal microscope is used to determine the hydrodynamic radius a of particles comparable to or larger than the linear size sigma of the confocal volume of the microscope, a correction must be used that depends on the a(2)/sigma(2) ratio and the distribution of the dye within the particle. Here we present the experimental validation of the theoretically predicted approximate correction necessary for appropriate measurements of the size of uniformly fluorescently labeled spheres of radius comparable to the size of the confocal volume. We also test the approximate correction formula for different ranges of the a/sigma ratio and propose a simple procedure to obtain the correct nanoparticle size from such a measurement.