New results of experimental investigation accounted for the parametric effects of the bubble size and the two-phase flow rates on the wall shear stress are reported. The mean and the time-varying fluctuation properties of the wall shear stress are measured based on a flush-mounted hot him sensor for air-water bubbly flow in a vertical channel. The unique feature of this study is that the experiments were carried out under various fixed gas and liquid fluxes, with only the bubble size being changed at the flow entrance. This has been made by using a special bubble generator to decouple the bubble size effect from the inlet condition. The test conditions cover both the wall and core peaking void distributions of bubbly two-phase flow. It is found that the wall shear stress is strongly influenced by the internal and wall proximity structure of the how, while both the liquid phase velocity and the wall concentrated bubbles are the dominant parameters on both the magnitude and the fluctuation intensity of the wall shear stress in the regime of bubbly flow. The present data are extensively compared with some other data sources and with the models used in predicting the wall shear stress. Due to no obvious existing wall shear stress data and models were accounted for the change in bubble size systematically, the present data thus may serve as a relatively complete comparative basis for the development of theoretical models.