Most of the existing constitute models of stirrup-confined concrete do not consider the size effect. A few consider the size effect using a strength reduction coefficient. In order to investigate the mechanical properties and size effect behavior of the large-sized stirrup-confined RC columns under axial compressive load, based on the experimental results of the circular and squared concrete columns, the influence of volume stirrup ratio, the arrangement of stirrups as well as the specimen size on stress-strain curves of confined RC columns are analyzed. Considering the influence of volumetric stirrup ratio and stirrup type, the size effect formula of peak strain for stirrup-confined RC columns was established. Moreover, combined with the size effect formula of peak stress in the previous study, the stress-strain model considering size effect for stirrup-confined RC columns was proposed. Through the comparison with the experimental and simulation data, it is demonstrated that the size effect formula of peak stress and peak strain show good consistency with the experimental results, and the stress-strain model provide satisfactory predictions in large-sized stirrup-confined RC columns.