Homogeneous clusters of aromatic molecules can emit fluorescence either at their van der Waals geometry or at their excimer geometry following structural rearrangements. The isomerization efficiency critically depends on both the cluster size and the excess vibrational energy deposited into each cluster. We propose here that the excited-state dynamics is a manifestation of the transition from weak to strong extremes of exciton interactions that accompanies the decrease in interchromophore separation and the increase in overlap. An S-2 exciton interaction model is presented which accounts for the size dependence of the excited-state dynamics in small naphthalene clusters. It is also shown that the isomerization dynamics is induced by vibrational excitation of cluster intermolecular modes. This excess energy dependence is most evident in the naphthalene trimer and explained in terms of the structural difference between the vdW and excimer geometries. The results of photodissociation and photoionization measurements on these clusters are discussed within the framework of this exciton interaction model.