We present a method for the evaluation of the average size of gold nanoparticles based on the fitting of their UV-vis spectra by the Mie model for spheres. The method gives good results using a calibration of the dumping frequency of the surface plasmon resonance and accounting for the presence of nonspherical AuNP in solution by the Gans model for spheroids. It has been successfully applied to free and functionalized gold nanoparticles in various solvents with diameters in the 4-25 nm range. Despite the differences among samples, we found an accuracy of about 6% on the nanoparticles average size with respect to sizes measured by transmission electron microscopy (TEM). Moreover, the fitting model provides other information not available from TEM like the concentration of AuNP in the sample and the fraction of nonspherical nanoparticles, which is particularly useful for measuring aggregation processes. The fitting procedure and models are thoroughly discussed in the text, and the fitting programs are freely accessible on the web.