Size effects influencing performance of ferroelectric thin film capacitors for memory applications are analyzed in relationship with the phenomena of depolarization, charge injection and inhomogeneous stress. The model describing the effect of ferroelectric film thickness on the switching properties is based on the electronic injection into the interfacial layer of the ferroelectric film in combination with the depolarization effects. A concept of stress- induced phase transition is evoked in order to explain the anomalous distribution of spontaneous polarization across the area of micron-size ferroelectric capacitors.