Surface smoothing is one of the most remarkable applications of large gas cluster ion beams (GCIBs), which consist of more than hundreds of atoms. The multiple collisions between constituent atoms and solid surfaces cause lateral sputtering, and remove the grains on the rough surface rapidly. Thus, GCIB irradiation can improve surface roughness at the atomic level with low fluence. This surface smoothing effect with GCIB has been reported on inorganic and organic samples, such as Cu, diamond-like carbon (DLC) and polymethylmethacrylate (PMMA). However, the roughness of irradiated surface was found to strongly depend on incident GCIB condition. For example, the surface roughness values of PMMA irradiated with 10 keV Ar₂₀₀₀ and 20 keV Ar₁₆₀₀₀ were 2.1 and 0.8 nm, respectively, although the sputtering yield with these ions were about the same. In molecular dynamics (MD) simulation studies, the surface roughness after irradiation with GCIB was found to depend on both the incident cluster size and energy-per-atom. Nonetheless, there are only a few reports on the optimum size and energy for gas cluster ion irradiation, because the size distribution of gas cluster extends over more than several thousand atoms and it is very difficult to investigate the effects of irradiation with clusters of a specific size. For a precise investigation, incident cluster size and energy-per-atom need to be controlled. In this study, we investigated the effects of incident cluster size and energy-per-atom on surface morphology with size-selected GCIB.