In this study, a TiO2 precursor solution was synthesized by the sol–gel method, and TiO2 nanoparticles of different sizes (5, 25, 32, and 100 nm) were added to investigate their effects on the properties of dye-sensitized solar cells (DSSCs). Thin films were deposited onto fluorine-doped tin oxide substrates via spin coating and subsequently sintered at 600 ℃. The effects of nanoparticle size on the surface morphology, optical characteristics, and photovoltaic performance of the resulting DSSCs were systematically studied. X-ray diffraction (XRD) was employed for structural analysis, while UV–VIS–NIR spectroscopy was used to measure the optical transmittance and absorbance of the films. The surface and cross-sectional morphology, grain size, and film thickness were examined by scanning electron microscopy (SEM). The current–voltage characteristics and conversion efficiency of the DSSCs were evaluated under simulated sunlight using a solar simulator. The experimental results revealed that smaller TiO2 nanoparticles generally led to improved photovoltaic performance. However, the film fabricated with 5 nm nanoparticles exhibited significant surface aggregation, which adversely affected the short-circuit current density (Jsc) and conversion efficiency, compared with the film fabricated with 25 nm nanoparticles. Among all the samples, the DSSC based on 25 nm TiO2 nanoparticles showed the best performance, featuring a uniform and smooth film morphology, Jsc of 14.49  mA/cm2, and a conversion efficiency of 6.05%.