The innovation of 3D printing technology in construction, particularly for components with complex geometries is revolutionizing modular construction. Shear keys, crucial joint components, ensure stability and structural integrity in modular assemblies, which are constructed from multiple smaller units due to the size limitations of prefabrication and 3D printing technologies. This study explores the impact of sizing variations on the shear behaviour of 3D-printed concrete shear keys, which are vital for efficient modular construction with concrete 3D printing. Employing an experimental approach, this research fabricated and tested shear key specimens with different dimensional variables to evaluate their performance under shear stress. These tests highlighted how different sizes influence crack patterns and shear behaviour, shedding light on the mechanical performance variability and structural implications of these components. Furthermore, a novel measurement technique using multiple laser sensors was applied, enhancing the precision of displacement measurements in small-sized specimens, suitable for studying size and dimensional effects in 3D-printed structures. Direct shear tests indicated that the average shear behaviour of smaller shear keys slightly underperformed compared to large ones, which contradicts the traditional notion that smaller components usually have greater mechanical behaviour than larger ones of the same shape. This finding suggests that the size of shear keys, which can be customized through 3D printing, significantly influences their shear strength, ductility and failure modes. This insight is crucial for optimizing 3D-printed joint components in real-world construction, aiming to enhance material efficiency and architectural flexibility. This study not only advances our understanding of 3D-printed construction components but also underscores the importance of tailored geometrical configurations to meet specific structural demands.