[Purposes] Due to aerodynamic interference between parallel bridges and the significant increase in the characteristic height of the bridge after installation of wind barriers, the bridge on the leeward side is prone to wake-induced vibration. It is necessary to study the size effect of wake-induced vibration response of parallel bridges for ensuring test accuracy and project safety. [Methods] Wind tunnel tests were conducted on parallel cable-stayed bridges with girder section models at scale ratios of 1∶20 and 1∶50. The effects of the damping ratio, wind attack angle, and girder shape on the size effect were discussed. [Findings] The girders of both bridges have significant size effects. Compared to the large-scale model of the railway bridge, its normal-scale model has a higher peak amplitude, its damping ratio and wind attack angle have a relatively smaller impact on the size effect of the wake-induced vibration response，but wind speed ranges ot the two models are basically similar. Compared to the large-scale model of highway bridge, the vertical and torsional vibration lock-in regions are wider, the amplitudes are larger in the normal-scale model of highway bridge. At a -3° wind attack angle, due to the diversion effect of the wind barrier of the railway bridge and the buffet effect of the normal-scale model, the peaks of vertical amplitude and torsional amplitude in the normal-scale model are lower than those in the large-scale model. [Conclusions] The size effect of wake-induced vibration varies depending on the cross-sectional dimensions of the same streamlined closed box girder.