In order to evaluate the effect of size on the adsorption energies on small Pt particles, we have performed a series of DFT-GGA calculations on Pt clusters of varying sizes, between 3 and 25 atoms, and a Pt(111) slab, with and without O and S adsorbates. We have found a significant variation in the energy of adsorption as a function of cluster size. In particular, energies of adsorption on the Pt-10 cluster showed the largest deviation from those on the slab surface. Structural and energetic properties compare excellently to published experimental data. The Pt-4 clusters with and without adsorbates were found to have reduced symmetry due to the Jahn-Teller effect. A simple rule between the cohesive energy and the size of small close-packed platinum particles is derived and confirmed by our first-principle calculations,, the extrapolation of which excellently matches both experimental and computational bulk cohesive energies.