Lateral size effects of ferroelastic domain structures in epitaxial PbTiO3 thin films were investigated systematically with a viewpoint of misfit strain relaxation mechanism. The epitaxial PbTiO3 thin films were patterned into discrete islands and the effects of lateral dimension were analyzed by reciprocal space mapping using synchrotron x-ray diffraction as well as finite element simulation. As the lateral two-dimensional planar size decreases in the PbTiO3 patterns on MgO(001), some of the a domains turned into c domains due to the relaxed tensile strain. In the PbTiO3 patterns on Pt(001)/MgO(001), on the other hand, the formation of 90° domains is enhanced by the reduction in compressive misfit strain. As the pattern size decreases further to 100 nm, the untilted a domains arise due to the almost completely relaxed misfit strains. Equilibrium domain structures in the epitaxial thin films and discrete islands are also analyzed by the finite element simulation and found to be consistent with the experimental observation. These results manifest that the domain structure and evolution of the epitaxial thin films could be engineered by the control of misfit strain and its relaxation.