Objective The mechanical properties of polymeric vascular stents considering size effect were investigated, and the influence laws of stent structure on mechanical properties of the stents and size effects during stent deformation were further analyzed, so as to provide the theoretical basis for structural design of the stents. Methods The Cosserat theoretical model of poly lactic acid (PLA) considering size effect was established, and combined with the finite element method, the bending stiffness and radial support stiffness of the stents were obtained by three-point bending and flat plate compression, and the influence laws of rib thickness and rib width of the stents, radius of curvature and axial spacing of the support unit on radial support performance and size effect of the stents were further analyzed. Results There was a significant size effect on the polymeric vascular stent during bending and compression. The radial support stiffness of the support unit was negatively correlated with the radius of curvature and axial spacing, and positively correlated with the rib thickness and rib width, and the smaller the radius of curvature and axial spacing of support unit, rib thickness and rib width of the stents, the larger the size effect during compression. Conclusions The radial support performance of the stent is mainly determined by the structure stiffness, and affected by the size effect during stent deformation. With smaller characteristic dimensions of the stent geometric structure and greater bending and torsional deformation of the stent, more obvious size effects will be obtained, which leads to a greater increase in the radial support performance of the stent.