The size dependence of ionization potentials, electron affinities and cohesive energies of small Hgn(n= 3,4,5,6,8,13, 15) clusters has been studied using a combination of relativistic energy-consistent pseudopotentials supplemented by core-polarization potentials and highly correlated valence wavefunctions. In order to ensure a size extensive treatment of electron correlation, coupled-cluster and quantum Monte Carlo calculations have been performed, and excellent agreement observed between the results obtained with the two methods. The experimentally observed systematic trends of these properties with respect to increasing cluster size are reproduced well.