Understanding phase transformation processes at the nanoscale is an important step for the integration of phase change materials into functional nanodevices. Here, a numerical method to assess surface energy and surface stress contributions to finite size effects in spin crossover nanomaterials is proposed. Starting from their formal definitions, molecular dynamic simulations combined with continuum mechanics and thermodynamic considerations on model thin films are performed. The surface energy values extracted from the simulations are 71 and 79 mJ m−2 for the high spin (HS) and low spin (LS) states, respectively. In the limit of a weak lattice parameter misfit, the calculated isotropic stresses of a HS/LS interface are 94 and 45 mJ m−2 for the LS and HS states, respectively. From these results, a ≈10 K downshift of the spin transition temperature is predicted with the size reduction. Whereas surface energy favors systematically the HS state, it is not the case for the interface stress. Indeed, it is demonstrated that the anisotropy of mechanical stresses can lead to the stabilization of the LS state. These results confirm the possible control of the phase stability in these smart nanomaterials through interface engineering.