Pressure fluctuations and X-ray computed tomography (CT) measurements were utilized to characterize the flow behavior of gas–solid fluidized beds using polyethylene particles in three Plexiglas columns with diameters of 10, 20, and 30 cm. Air was used as the gas phase. The gas–solid flow dynamic under ambient conditions was characterized from statistical analysis of pressure fluctuation data and CT images. The time-averaged voidage distribution, bubble-phase area fraction, bubble diameter and bubble number distribution varying with the bed heights were extracted from all the three columns. The bed scales had significant effect on the hydrodynamics. The scale-up effects on the gas–solid two-phase flow behavior were discussed.