The electron spin relaxation processes are complicated in semiconductor quantum dots. Different spin relaxation mechanisms may result in an increased or decreased spin relaxation rate with the size. The information on size-dependent spin dynamics helps to clarify and better understand the underlying spin relaxation processes. We investigate the size dependence of the electron spin dynamics in negatively photocharged CdSe and CdS colloidal quantum dots by time-resolved ellipticity spectroscopy. It is revealed that the electron spin dephasings of photodoped electron in zero or weak magnetic fields are dominated by the electron-nuclear hyperfine interaction for all measured samples. The hyperfine-induced electron spin dephasing time is ∼1-2 ns at room temperature and decreases with decreasing the size D. In addition to a size-dependent dephasing time that is directly proportional to D3/2, our measurements also show a size-independent time component, likely due to the laser-induced nuclear spin ordering.