Systems containing both argon cluster with number of atoms N = 100, 200, or 400 and argon vapor were studied with constant energy molecular dynamics simulations. The vapor pressure versus temperature phase diagram including the melting temperatures was determined as a function of cluster particle number. The melting temperature of the cluster was determined with a new method based on the nearest-neighbor exchange of atoms, and it was found to approach the bulk melting temperature linearly as a function of N-1/3. The vapor pressure versus temperature curve approached the corresponding bulk curve as the particle number of the cluster increased. At temperatures lower than the melting temperatures the clusters were found to contain an icosahedral core.