As is well-known, fatigue limit, threshold stress intensity range and fatigue crack growth rate are influenced by the specimen or structure size. Limited information on size effect is available in the literature. In the present paper, by employing some concepts of fractal geometry, new definitions of fatigue limit, fracture energy and stress intensity factor, based on physical dimensions different from the classical ones, are discussed. Then, size-dependent laws for fatigue limit, threshold stress intensity range and fatigue crack growth rate are proposed. Some experimental results are examined in order to show how to apply such theoretical scaling laws.