The scenarios of the size effects which may account for the difference in the switching properties of ferroelectric thin films and bulk ferroelectrics are critically reviewed. It is shown that the two often used models (the Janovec-Kay-Dunn surface nucleation size effect and the dielectric layer model) are irrelevant to the experimentally observed thickness dependence of the coercive field in the films. The surface pinning model and the depletion-assisted nearby electrode nucleation model are shown to be consistent with the experimental observations. It is shown that the Miller-Weinreih law for wall velocity cannot be expected in thin films.