A simple and unified model, free of any adjustable parameters, is developed for the finite size effect on glass transition temperatures of polymers and organic particles. As the thickness of polymer thin films and the radius of organic particles decrease, their glass transition temperatures decrease. For polymers, this decrease is independent of their molecular weight, but dependent on the correlation length for intermolecular cooperative rearrangement and the presence of substrates. The model predictions are consistent with available experimental results on size dependence of the glass transition temperatures for free-standing polystyrene thin films, polystyrene films supported on passivated substrates and o-terphenyl and benzyl alcohol nanoparticles.