To study the size effect of reinforced concrete columns strengthened with carbon fiber reinforced polymer (CFRP) under axial compression loading, three groups of thirty geometrically similar reinforced concrete columns strengthened with CFRP were designed and tested. Test parameters including column size, reinforcement layout, and loading history were studied, and the relationships between the properties of the proposed columns (such as failure mode, ultimate strength, peak stress, and residual deformation) and column size were investigated under axial compression loading. Experimental results show that with the same size, the ultimate strength of the reinforced concrete columns strengthened with CFRP was higher than that of the plain concrete columns. As the size of the concrete columns strengthened with CFRP increased, the peak stress first increased and then decreased. The normalized axial deformability and residual deformation of the concrete columns strengthened with CFRP decreased with the increase in the size of the columns. Different loading histories had little effect on the bearing capacity and ultimate displacement of the concrete columns strengthened with CFRP.