In recent years, the dry reforming of methane (DRM) reaction has gained widespread attention due to its effective utilization of two major greenhouse gases. Supported Ni-based catalysts for DRM exhibit a strong dependence on particle size, however, the reaction mechanisms involved remain unclear. In this work, the effect of metal particle size on CO₂ activation and CO formation was explored in the DRM reaction using the density functional theory. Nix/MgO (x = 13, 25, 37) was constructed to investigate the CO₂ activation and the formation of CO during the DRM reaction. It is found that CO₂ is more inclined to undergo chemisorption on Nix/MgO before activation. With the variation in particle size, the main activation pathway of CO₂ on the catalyst changes. On the smallest Ni13/MgO, CO₂ tends to directly dissociate, while on the larger Ni25/MgO and Ni37/MgO, the hydrogenation dissociation of CO₂ is more kinetically favorable. Compared to Ni13/MgO and Ni37/MgO, the oxidation of surface C atoms and the oxidation of CH occur more readily on Ni25/MgO. This indicates that C atoms are less likely to form on Ni25 particle and are more easily to be oxidized. To some extent, the results suggest that Ni25/MgO exhibits superior resistance to carbon formation.