Abstract: A total of 12 geometric similar stirrup-confined circular reinforced concrete (RC) columns subjected to axial compression were designed and tested. In the tests, two stirrup ratios of 1.26% and 2.89% were utilized. The structural similarity ratio of the columns was 1:1.5:2.25. The maximum cross-sectional diameter was 576 mm, and the slenderness ratio was 3. The confinement effect on the failure patterns and the axial compressive strength of the stirrup-confined columns was explored. Moreover, based on the basic theory of the Size Effect Law (SEL) proposed by Bažant, a modified formula for evaluating the bearing capacity of stirrup-confined RC columns was developed, which can reflect the effect of structural size. Good agreement between the theoretical results and the test results indicated the reasonability of the modified formula. Finally, a 3D meso-scale numerical method for the simulation of the failure behavior and size effect of RC members was established. The simulated results were consistent with the tested ones, and the meso-scale simulation method was then extended to model the failure of circular RC columns with larger structural dimensions.