The effects of polydispersity of size on the average static structure factors of colloidal dispersions with short-range repulsive forces and long-range attraction are examined using the solution of the multicomponent Ornstein-Zernike equation under the so-called HMSA closure. The accuracy of the HMSA closure is examined using structure factors evaluated directly in Monte Carlo simulations. The results show that the structure factors can be approximated sufficiently accurately using the structure factor based on a monodispersed dispersion consisting of particles of diameter equal to the mean diameter if the standard deviation of the distribution is less than about 15% of the mean diameter. It is also shown that, although size-polydispersity can affect the configurational distributions significantly, the specific form of the size distribution is not important even at standard deviations as high as 30% of the mean size. The results presented here provide the first test of the HMSA closure for a multicomponent (i.e., one with more than two components) system.