The effects of bubble size on an upward gas-liquid (CO2-water) flow in a vertical pipe after an abrupt expansion is investigated visually and experimentally in the present work. The mean bubble size varies from small scale (db=0.3mm) to relatively large scale (db=4.5mm). Extensive visualization experiments and PIV analysis show different flow patterns downstream of the expansion of flows containing bubbles of different sizes. The effect of bubble size is also investigated measuring the pressure distribution along the pipe and the drag of the expansion and its difference under different bubble sizes is calculated and compared with that of single-phase flow. The fluctuation phenomena occurring downstream are also investigated. The experiments are conducted under constant Reynolds number (Re=1.0×104) and volumetric gas flow rate ratio (αv=0∼10%). The present work gives valuable information about how the bubble size affects the flow characteristics even under steady flow conditions, and explains the differences between results reported by other authors investigating under similar conditions.