The shear failure of rock masses is one of the primary causes of underground engineering instability. The shear mechanical behavior of rocks at different sizes is of great significance for studying the shear failure pattern of engineering rock masses. However, due to the presence of various joints and defects in natural rocks, the obtained rock specimens exhibit significant discreteness, making it difficult to customize specimen sizes for size effect studies. In recent years, 3D printing (3DP) technology has gained widespread application in rock mechanics test due to its high printing precision, ability to form specimens in a single step with minimal discreteness. Among these, specimens prepared using sand-powder 3DP exhibit elastoplastic mechanical characteristics similar to those of natural rocks. Therefore, this study utilized sand-powder 3DP to prepare rock-like specimens in four different sizes and conducted compression-shear tests under three different shear velocities. The shear strength, shear strain, and wear of the shear surfaces were analyzed as functions of specimen size and shear velocities. The results indicate that, under the same shear velocity, the shear strength of the specimens is negatively correlated with specimen size; The peak shear strain is generally unaffected by shear velocities, but it increases initially and then decreases with increasing specimen size; As specimen size increases, the degree of specimen damage intensifies, and larger specimens are more prone to developing derived fractures. The study's findings contribute to expanding the applicability of sand-powder 3DP in the research of shear mechanical properties of soft rocks and provide a critical basis for accurately and scientifically selecting appropriate rock specimen sizes for laboratory tests.