We analyzed the size effect of spin-crossover transition nanoparticles in a two-dimensional core-shell model, where the edge atoms are constrained to the high-spin (HS) state. Using Monte Carlo (MC) simulations, we showed that this specific edge effect lowers the equilibrium temperature and enhances the HS residual at low temperature; these results are in very good agreement with recent experimental data. Within a very simple working assumption, we obtained an analytical expression for the size dependence of the equilibrium temperature that is in excellent agreement with the MC results. The model leads to a nontrivial size dependence of the hysteresis width, which is similar to a-size-dependent-negative pressure effect induced by the HS edges. To reach the best agreement with experimental data, we accounted for the size distribution of the experimental samples. © 2011 American Physical Society.