The correlation between the strain, spin disorder, and the thickness dependence of the magneto-transport has been investigated in as-grown and post-annealed 25-300-nm thick films of Sm_0.55Sr_0.45MnO₃ (SSMO) deposited on SrTiO₃ (STO) and LaAlO₃ (LAO) substrates. In the post-annealed SSMO/LAO films, the epitaxial strain increases with a decreasing thickness; however, it is independent of the thickness of the post-annealed SSMO/STO films. In both cases, the reduction of the metal-insulator transition temperature with a decreasing thickness was observed. This behavior is attributed to an increasing compressive strain in the SSMO/LAO films and to spin disorder effects in the SSMO/STO ones.