This study investigated the shear behavior of reinforced concrete (RC) beams strengthened with externally bonded (EB) fiber-reinforced polymer (FRP) strips, focusing on the size effect and the shear contribution of both concrete and FRP strips. Small-, mid-, and large-scale RC beams were tested to evaluate the shear strength gain and the effective strain of FRP strips. The results show that the shear strength gain decreases as beam size increases, indicating a size effect. Additionally, the shear strength gain is not equal to the shear contribution of the FRP strips, suggesting that an interaction between concrete and FRP strips exists. The effective strain of the FRP strips decreased with increasing beam size, corresponding to 37%, 30%, and 25% of the ultimate tensile strain for the small, mid, and large-scale beams, respectively. Some design equations predicted effective strains that aligned with the experimental findings, while equations from ACI 440 and the Concrete Society TR55 showed a contradictory trend, suggesting a need for revision. These results highlight the importance of considering the beam size effect and the interaction between concrete and FRP strips in the development of design equations and guidelines for the shear strengthening of RC beams with FRP systems. Finally, a finite element (FE) model was developed to predict the behavior of RC beams strengthened with EB FRP. This model will be used for further parametric studies involving different beam sizes, steel reinforcement details, and FRP strengthening configurations to refine the findings of this study.