The coeffect of size and stress on the properties of Bi3.25La0.75Ti3O12 thin films was studied. With the applied stress changing from maximum compression (about -80 MPa) to maximum tension (about 80 MPa), the remnant polarization increases while the coercive field decreases. The variational extent of the remnant polarization with stress is bigger, while that of the coercive field is smaller in samples with smaller grains. Meanwhile, fatigue properties of films can be improved by stress (especially compression), and the improvement increases with increasing grain size. From these results, the coeffect of size and stress on domain patterns and domain wall movability was deduced.