Airborne dust is an important constituent in the Martian atmosphere because of its radiative interaction with the atmospheric circulation. Dust size is one crucial factor in determining this effect. In reality dust sizes are varied; however, in numerical modeling of dust processes, dust size has usually been described by choice of a particular size distribution function, or by use of fixed values of effective radius (ER) and effective variance (EV). In this work, we present analytical expressions that have been derived to specify ER and EV for N-bin dust schemes, based on a model-calculated dust mixing ratio. Numerical simulations based on this approach thus would consider the effects of variable ER on the atmospheric radiation and their interaction. Results have revealed some interesting features of the dust distribution parameters, such as seasonal and spatial variation of ER and EV, which are generally consistent with some previous observational and modeling studies. Compared with the usual approach of using a fixed ER, simulation results from the present approach suggest that the variability of ER can have significant effects on the simulated thermal field of the Martian atmosphere.