The effect of the intermicellar exchange on the size control of nanoparticles synthesized in microemulsions has been studied by computer simulation. The employed algorithm includes a reactant exchange parameter, k, which depends on the surfactant film flexibility and the dimer lifetime. This parameter, k, has a big influence on the final particle size, except in the case that one of the reactants is in a big excess. It has also been proven that a critical value of the reactant exchange parameter exists, which determines the relative weights of the two mechanisms of growth: autocatalysis and ripening. It is then predicted that there is an optimum dimer lifetime for which the smallest particles can be obtained.