Recent experiments on the thinning of liquid films formed from colloidal suspensions carried out in our laboratory [Nikolov, A., and Wasan, D. T., J. Colloid Interface Sci. 133, 1 (1989); Langmuir 8, 2985 (1992)] have revealed the pronounced effect of the presence of bidisperse-sized particles on the film stability. To shed more light on the role of the size bidispersity on the film properties, we performed a statistical mechanics study of a model system that consists of a binary hard-sphere mixture with a size ratio of 1: 10 between two planar surfaces. The variations of the particle local densities, excess surface coverage, film disjoining pressure, and film interaction energy per unit area with separation between the surfaces in the presence of monodisperse and bidisperse particles have been investigated. A repulsive depletion interaction (depletion stabilization) between the surfaces in the case of a bidisperse suspension has been observed at separations of about one layer of the larger particles.