The impeller configuration with a hollow half-elliptical blade dispersing turbine below two up-pumping wide-blade hydrofoils, identified as HEDT+2WH(U) and recommended in previous work, was used in this study. The effect of the top impeller diameter, ranging from 0.33T to 0.50T, on gas-liquid flow in a stirred tank of 0.48 m diameter was investigated by experimental and CFD methods. Power consumption, total gas holdup, and local void fraction were measured for the impeller configurations with different top impeller diameters. Results show that while the ratio of top impeller diameter to tank diameter (D-top/T) increases from 0.33 to 0.50, the relative power demand (RPD) in a gas-liquid system decreases slightly. The increase of total gas holdup with rising gas flow rate becomes less evident as D-top/T increases from 0.33 to 0.50. Local void fractions at the measurement points above the height of 0.8T increase significantly with the increase of top impeller diameter. When D-top/T = 0.50, there is an extremely large voidage of about 50% just above the top impeller. The maximum voidage decreases with the decrease of top impeller diameter and almost disappears when D-top/T = 0.33. CFD simulation based on the two-fluid model along with the population balance model (PBM) was used to investigate the effect of top impeller diameter on the gas-liquid flows. The power consumption, total gas holdup, and local void fraction predicted by the CFD approach were in reasonably good agreement with the experimental data.