A theoretical model was developed in order to predict the size of agglomerates ejected from aluminized composite solid propellants. The model accounts for aspects of both mechanistic and geometric approaches. A thin mobile surface layer is assumed to dominate particle accumulation. Agglomeration number N-ag expressing the ratio between ignition and accumulation times, was found to greatly affect the characteristics of agglomeration phenomena, as noted in previous works. The concept of a characteristic distance D-e of the mobile surface layer was introduced, depending mostly on coarse AP particle diameter and loading. It was found to have a significant effect on the resulting agglomerate diameter. An agglomeration threshold was defined as the particle diameter equal to the mobile layer thickness. Particles smaller than this threshold have shown tendency to form large agglomerates, whereas particles with a somewhat larger diameter have demonstrated faster ignition and smaller ejected agglomerates. Model predictions for a number of propellant compositions showed good agreement with other theoretical studies as well as with experimental results.