The consequences of the size polydispersity for the phase diagram of ionic ferrofluids are examined by considering a mixture composed of colloidal particles of different sizes. Because of the experimental fact that the ionic strength in the colloid-rich phase is similar to that in the colloid-poor phases, we do not consider explicitly the suspending solution. We thus consider a continuum model for the suspending solution, where one deals with osmotic properties. Moreover, from the experimental constraint that the nominal (or overall) size distribution function is independent of the nominal volume fraction in colloidal particles, we deduce that the number of different sizes taken into account is not a relevant variable of the problem, and we thus consider the simple case of a binary mixture. Our model is a mixture of Stockmayer particles and we use a perturbation theory to determine the free energy from which the two-phase coexistence is obtained. Our results are in qualitative agreement with experiments.