In this paper, we study how pixel size influences energy resolution for a proposed pixelated detector—a high sensitivity, low cost, and real-time radon monitor based on a time projection chamber (TPC). This monitor was designed to improve spatial resolution for detecting radon alpha particles using sensors assembled by a 0.35 μm CMOS integrated circuit process. Owing to conces that small pixel size might have the side effect of worsening energy resolution due to lower signal-to-noise ratio, a Geant4-based simulation was used to investigate the dependence of energy resolution on pixel sizes ranging from 60 to 600 μm. A non-monotonic trend in this region shows the combined effect of pixel size and threshold on pixels, analyzed by introducing an empirical expression. Pixel noise contributes 50 keV full-width at half-maximum energy resolution for 400 μm pixel size at 1–4r threshold that is comparable to the energy resolution caused by energy fluctuations in the TPC ionization pro-cess (∽20 keV). The total energy resolution after com-bining both factors is estimated to be 54 keV for a pixel size of 400 μm at 1–4r threshold. The analysis presented in this paper would help choosing suitable pixel size for future pixelated detectors.