The scenarios of the size effects which may account for the difference
in the switching properties of ferroelectric thin films and bulk ferr
oelectrics are critically reviewed. It is shown that the two often use
d 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 p
inning model and the depletion-assisted nearby electrode nucleation mo
del are shown to be consistent with the experimental observations. It
is shown that the Miller-Weinreih law for wall velocity cannot be expe
cted in thin films.