The absorption bandwidth of a metasurface absorber can be adjusted by varying the thickness of the substrate, shape of the conductive pattern attached on top of the substrate, or conductivity of the pattern. In this study, the bandwidth of a pixelated metallic metasurface absorber was analyzed with respect to its unit cell size. The genetic algorithm (GA) was used to determine the best combination of the metallic pixels among 16 × 16 square pixels for each unit cell size. The size of the unit cell was adjusted around the 1/2 wavelength, which guarantees the suppression of the grating lobe for the normal-incidence condition. Based on full-wave simulation results, the −10 dB reflectance bandwidth was expanded as the size of the unit cell was increased from 3/8 to 3/4 wavelength with an interval of 1/8 wavelength at the center frequency of 5.8 GHz. The results demonstrate that the absorption bandwidth of the metasurface absorber is expanded by increasing the size of the unit cell around the 1/2 wavelength.