The concept of dispersion self-energy of a molecule of finite size is used to study the role of dispersion forces in physical adsorption on dielectric surfaces. The adsorption energy, given in terms of readily accessible macroscopic data, remains always finite, enabling the removal of a phenomenological parameter which occurs in previous theories. Using a statistical theory similar to that of Brunauer, Emmett and Teller, the Lifshitz theory of thick films is shown to emerge as a special case, and its region of validity studied. Restricted adsorption and the Hill equation arise as special cases of the theory. The influence of molecule size on adsorption processes is built in automatically.