We report on the thickness dependence of the ferroelastic domains of PbTiO3 films grown on (110)-DyScO3 with low thicknesses (up to 240 nm), which fall outside the validity range of the square root law proposed by Roytburd. For slow-grown films, the data reveal the linear thickness dependence predicted by Pertsev and Zembilgotov, using a complete elastic description, while a 2/3 scaling exponent is found for fast-grown films. Extremely long domains running all through the samples have been observed in the latter case, compared to the short domains observed in slow-grown films. These differences are ascribed to the in-plane anisotropy for domain wall nucleation, which is likely caused by the anisotropic elastic modulus of the substrate.