The inverse problem of optical sizing of ultrafine metallic particles from the spectral extinction measurements in the visible range is investigated. Solving the inverse problem becomes possible due to the strong size effect which in the framework of classical electrodynamics can be described by the dependence of complex refractive index on the particle size. It is shown that the size effect leads to the considerable increase of information content of spectral extinction data with respect to desired size composition of the particles. This makes it possible to retrieve the size distribution of ultrafine metallic particles with reasonably high accuracy, including the Rayleigh size region. The analysis is performed mainly within the framework of numerical tests by the typical example of ultrafine silver particles in a gelatin matrix. The results in retrieving of size distribution from experimentally measured extinction spectra are also presented. Calculations of spectral extinction coefficient are made by means of Mie theory. In so doing, the dielectric function of particles is modified by using the electron's mean free path limitation model,