An evaluation of nanoparticle size effects on the critical heat flux (CHF) was conducted through pool boiling experiments under atmospheric pressure in low concentration regions from 0.1 to 10 ppmv. A smooth flat-type heater made of stainless steel was used as a test section. Silica nanofluids were prepared by sonication processes to guarantee stability using various sizes of silica nanoparticles (8, 15, 65, and 400 nm). In the experiments, we observed the CHF enhancement at high concentration regardless of particle size. However, the deterioration of the CHF was observed in the relatively low concentration regions with particles that were smaller than 100 nm. Furthermore, when the particle size decreased, the CHF values appeared to be higher at the high concentration of 10 ppmv. On the other hand, for large particles with a size of 400 nm, no deterioration of the CHF at low concentrations was observed. The boiling time, which is considered to affect the nanoparticle deposition, had marginal influences on CHF values. Based on the CHF trends, surface analyses with heaters after the experiments were conducted through SEM images, and contact angles and roughness values were used to evaluate the surface characteristics modified by nanoparticle deposition. Furthermore, in this study, the discussions on the particle size effects and deterioration of the CHF, and also its enhancement, are included based on the behavior of the structural disjoining pressure, nucleation site density, and porous layer as a function of the nanoparticle size and concentration.