In this study, we performed density functional theory based calculations to determine the effect of the size of Cux (x = 1 (adatom), 3 (trimer), 7 (heptamer)) clusters supported on Cu(111) toward the adsorption of CO, O, and CO2, and the dissociation of CO2. CO adsorbs with comparable adsorption energies on the different cluster systems, which are influenced by the reactivity of the Cu atoms in the cluster and the interaction of CO with the Cu atoms in the terrace. The O atom, on the other hand, will always favor to adsorb on hollow sites and is more stable on the hollow sites of smaller clusters. CO2 dissociates with lower activation energy on the cluster region than on flat Cu(111). We obtained the lowest activation energy on Cu3 due to its more reactive Cu atoms than the Cu7 case and due to the possibility of O to adsorb on the cluster region, which is not observed in the Cu1 case. The presented results will provide insights on future studies on supported cluster systems and their possible use as catalysts for CO2-related reactions.