In the actual stress state, reinforced concrete (RC) columns/beams are subjected not only to axial force and bending moment but also, possibly to torque moment. Under combined loading including torque moment, the failure of structures is more serious and difficult to predict. The torsional failure has a significant size effect because it often exhibits brittle failure characteristics. The torsional structure design of RC members has been widely concerned in recent years. However, the existing design codes are extrapolated based on the test data of small size specimens without considering the impact of size effect, so the safety degree of the predicted values of large size specimens remains to be further discussed. In this paper, the size effect law of nominal torsional strength of RC columns was summarized, and the size effect law formula of nominal torsional strength which can quantitatively reflect the effect of stirrup ratio was established. Furthermore, by introducing the size effect coefficient αh, a suggestion predicted formula for pure torsional capacity of RC members considering the influence of size effect was proposed to ensure the safety of the predicted values of large size specimens. By comparing the existing test data, it was proved that the modified formula can effectively improve the safety reserve of the predicted bearing capacity of large size specimens.