<p>In this study, the size-dependent dynamic behavior of micro-pipes conveying fluid under the influence of a moving harmonic load is examined by considering the surface effects. To accurately capture the influence of size-dependent effects, the modified strain gradient theory (MSGT) is employed in conjunction with the Knudsen number. The MSGT incorporates three length scale parameters, enabling a comprehensive analysis of small-scale effects. Utilizing the Timoshenko beam model in conjunction with a longitudinal magnetic field, the governing equations and boundary conditions that account for size dependency are derived. To assess the dynamic displacement, the Galerkin and differential quadrature methods (G-DQMs) are applied. The numerical results are compared with existing literature, demonstrating a strong correlation with previously published findings. The influence of various parameters, including the material length scale, the velocity of the moving harmonic load, the longitudinal magnetic field, surface effects, boundary conditions, and fluid velocity on the dynamic response of the micro-pipes, is explored in the numerical results. The analysis reveals that these parameters play a crucial role in understanding the forced vibrations of micro-pipes. Notably, the findings indicate that the MSGT predicts a smaller maximum dimensionless dynamic displacement and a higher critical load velocity compared to both the modified couple stress theory (MCST) and classical theory (CT). This underscores the significance of incorporating size effects in the dynamic analysis of micro-structured systems.</p>