Si nanocrystals (nc-Si) with different sizes embedded in SiO2 matrix have been synthesized with various recipes of Si ion implantation. The influence of nanocrystal size on optical properties, including dielectric functions and optical constants, of the nc-Si has been investigated with spectroscopic ellipsometry. The optical properties of the nc-Si are found to be well described by the four-term Forouhi-Bloomer model. A strong dependence of the dielectric functions and optical constants on the nc-Si size is observed. For the imaginary part of the dielectric functions, the magnitude of the main peaks at the transition energies E1 and E2 exhibits a large reduction and a significant redshift in E2 depending on the nc-Si size. A band gap expansion is observed when the nc-Si size is reduced. The band gap expansion with the reduction of nc-Si size is in good agreement with the prediction of first-principles calculations based on quantum confinement.