Using ab-initio computational modeling, we studied the effect of particle size on Pt dissolution in acidic aqueous solution. We constructed Pourbaix diagrams of bulk and Pt nanoparticles with varying sizes (0.5, 1 and 2 nm) and surface chemisorption of O and OH. Our results show that the stability regions of Pt metal, PtOx and Pt(OH)x are strongly dependent on the particle size, operating potential and pH of the solution. We found that the loss of cohesive energy in the nanoparticle leads directly to enhanced dissolution, compared to bulk Pt. Also, the solid-aqueous phase boundary moves to higher potentials for larger nanoparticles, an effect that induces particle coarsening when cycling the potential over that phase boundary. Our results indicate that a vital step towards long-term functioning fuel cells is to increase the inherent stability of the nanoparticle Pt against dissolution.