This study thoroughly investigated the synthesis and application of silica nanoparticles (nanosilica) with controlled sizes to enhance the performance of cementitious mixtures. While nanosilica has been widely studied for its potential to improve cementitious mixture properties, existing studies typically employed commercially available nanosilica with a particular particle size, leaving the effect of particle size variation largely unexplored or resulting in inconsistent or controversial findings regarding its effect on performance. To address this gap, nanosilica were synthesized in-house, enabling precise control over particle size. Techniques such as transmission electron microscopy, dynamic light scattering, ζ-potential, X-ray diffraction, Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy were employed to characterize the prepared nanosilica samples. The study focused on four distinct particle sizes (10, 35, 65, and 90 nm) to cover the whole range of nanoscale and systematically investigated the compressive strength, hydration, and rheological properties of cement paste incorporating nanosilica at different concentrations of 1, 2 and 3 wt%. Microstructural analyses, including thermogravimetric analysis and quantitative X-ray diffraction, were also conducted on the selected cement pasts incorporating nanosilica to shed light on the mechanisms underlying the performance of nanosilica in the paste. The findings revealed that the smallest particle size of nanosilica (10 nm) provided the highest compressive strength enhancement (over 100% enhancement when used at 2 wt% of cement), surpassing the reported data for the commercially available nanosilica available in the literature. The enhancing effects of the nanosilica particles on the compressive strength of the pastes were less substantial when their particle size increased from 10 to 90 nm. This study provided valuable insights into the effects of nanosilica particle size on the properties of cementitious mixtures, offering a potential pathway for the development of advanced construction materials with superior performance.