Segregation of impurities to grain boundaries in materials impact their structural, mechanical, electrical or functional properties. In the present paper, a theoretical study of the influence of grain size on grain boundaries segregation in a binary system by coupling the McLean theory, the concept of excess number with the grain boundaries fraction, is presented. The obtained results show that both grain boundary and bulk solute content decrease with reducing the grain size. However, it is shown the equilibrium segregation rate to grain boundaries increases. This effect is enhanced when the grain size is lower than 10 mu m and thus should be amplified in nanomaterials. This size dependence is larger at low temperatures, where segregation becomes more pronounced.