Many advanced industrial and biomedical applications that use silver nanoparticles (AgNPs), require that particles are not only nano-sized, but also well dispersed, not aggregated and not agglomerated. This study presents two methods able to give rapidly sizes of monodispersed AgNPs suspensions in the dimensional range of 20-100nm. The first method, based on the application of Mie's theory, determines the particle sizes from the values of the surface plasmon resonance wavelength (SPR), read from the optical absorption spectra, recorded between 190nm and 800nm. The computed sizes were compared with those determined by transmission electron microscopy (TEM) and dynamic light scattering (DLS) and resulted in agreement with the nominal values in a range between 13% (for 20nm NPs) and 1% (for 100nm NPs), The second method is based on the masterly combination of the Sedimentation Field Flow Fractionation (SdFFF - now sold as Centrifugal FFF-CFFF) and the Optical Absorption Spectroscopy (OAS) techniques to accomplish sizes and quantitative particle size distributions for monodispersed, non-aggregated AgNPs suspensions. The SdFFF separation abilities, well exploited to size NPs, greatly benefits from the application of Mie's theory to the UV-vis signal elaboration, producing quantitative mass-based particle size distributions, from which trusted number-sized particle size distributions can be derived. The silver mass distributions were verified and supported by detecting off-line the Ag concentration with the graphite furnace atomic absorption spectrometry (GF-AAS).