Due to the strong magneto-elastic coupling in ferromagnetic/ferroelectric heterostructures and the potential applications in low-power magnetoelectric nanodevices, ferroelastic domains and the corresponding dynamic evolution under external stimuli have attracted intense research interest. Using pulsed laser deposition method, we have successfully grown layered-perovskite Bi₂WO₆ thin films on SrTiO₃ (001) substrates. Interestingly, for the as-grown thin films with step-flow morphology, the relationship between ferroelastic domain number and size shows a normal distribution, which is similar to the Boltzman distribution for confined gas molecules at equilibrium. In addition, with post-annealing, the thin films with as-grown island-like morphology can be optimized for layered morphology, and the initial small ferroelastic domains can grow into large domains. This study provides an effective strategy for ferroelastic domain engineering, which can be applied for the design of multiferroic heterostructures and low-power nanodevices.