<p>Investigating the scale effect on the shear behavior of rock joints is pivotal for bridging laboratory studies with engineering applications. The scale effect significantly influences the shear response of rock joints, resulting in variability and uncertainty in their mechanical behavior. To overcome the limitation imposed by laboratory sample sizes, numerical simulations were employed to investigate the influence of scale on peak shear strength, shear stiffness, and peak shear displacement. In this study, the effective joint samples were obtained based on the overlapping sampling method, and the numerical direct shear tests were performed using the distinct element method. The results reveal two primary scale effect patterns: an exponential decay and a linear increase. Specifically, both peak shear strength and shear stiffness exhibit an exponential decay trend, stabilizing for joint sizes exceeding 50 cm and 130 cm, respectively, with errors remaining below 0.01. In contrast, peak shear displacement increases linearly, characterized by a slope of 0.04, thereby providing a robust means to predict the critical sliding displacement of unstable rock masses. The validity of these scale effect patterns was further confirmed through a case study in Shaoxing, Zhejiang Province, China. Additionally, a comprehensive discussion is presented on the criteria for selecting sampling locations to ensure representative outcomes. This work thereby enhances the precise assessment and prediction of the shear behavior of rock joints.</p>