The decrease in the nominal failure stress of geometrically similar but different sized members with the increasing characteristic size is called size effect. This effect occurs not only in tension failures but in compression failures as well. This is the case for prestressed concrete members and columns as well. In this study test results of full-scale failure of singly reinforced four-point-bend beams of different sizes containing deformed longitudinal reinforcing bars are reported. A residual stress in brittle specimens are recorded which are modelled and inserted in Bazant's size effect law by using Marquardt-Levenberg algorithm by a short program prepared in Matlab®. Two methods are compared with each other. This study indicates a significant size effect which should be considered in behaviour and design of structures and Bazant's modified size effect law is more successful than the Bazant's former law in greater specimens because of residual stresses.