A model of flame propagation based on the stationary flame formulation is employed to investigate the effect of size and size distribution on the burning velocity of a polydisperse spray. A quasi-linearization technique is introduced with which the behavior of the spray deflagration can be correlated in terms of its mean droplet size and degree of dispersity for the first time. Both the monodisperse and polydisperse cases are discussed. Results are in good agreement with the established experimental and theoretical data. This present work is part of a series program which aims at the fundamental study of the mechanism in the process of flame propagation in heterogeneous fuel sprays.