The size effects of the critical behaviors for the systems of interacting spins are discussed extensively in literature. In this paper, the finite-size dependence of the critical temperature and susceptibility of the ferroelectric thin film are investigated numerically based on the four-state Potts model with the nearest-neighbor interactions between the dipole moments. The four orientations of the domains exist in the ferroelectric film and the movement of the domain walls determines the polarization switching process besides the boundary conditions of the film. The critical exponents are obtained and our investigations show that the boundary conditions play the important roles for the ferroelectric properties of the thin films and the critical behavior of the thin films strongly depends on the feature of the surface.