The spin-crossover phenomenon in nanomaterials has been the subject of exploratory investigations for stimuli-responsive switching properties at nanoscale. Using variable-temperature Raman spectroscopy, we investigated changes in the temperature-driven spin-transition property of {Fe(py)2[Pt(CN)4]} (py = pyridine) induced by a size reduction from a bulk polycrystalline powder to an ultrathin film (crystallite size, 15 nm). When the crystallite size was reduced, the spin-transition temperature shifted lower and the spin-transition curves became less steep. In the thin-film form, the residual high-spin fraction was found to be about 20%.