The interaction of a powerful laser beam with a target material may result in the formation of a plume, which consists of gas molecules and molecular clusters directly ejected from the surface. A hybrid Molecular Dynamics - direct simulation Monte Carlo scheme capable to model the formation and evolution of the multi-component plume has been proposed recently. In the present study, the first objective is precise characterization of various types of collisions and other reactions among the molecules and clusters with the aid of molecular dynamics simulations. The updated scheme is then applied to study the effect of laser spot size on the evolution of the plume. The increased number of reactions per particle for larger spot size smoothed out a typical two-fold monomer density profile observed for smaller spot sizes.