The influence of the packing size on the mean bubble diameter in a flooded bed reactor packed with glass spheres of three different diameters (1.9, 4.0, and 9.3 mm) was investigated. The gas saturation and interfacial area were respectively measured by electrical capacitance tomography and chemical absorption of oxygen. It is observed that the presence of packing has greatly reduced the bubble diameter and increased the gas saturation compared with a bubble column. To explore the underlying relationship between the bubble diameter and packing size, the packing size featured by the pore diameter was utilized as a reference. It shows that the bubble diameter is 2-3 times the pore for the 1.9 and 4.0 mm packings, while they are approached for the packing of 9.3 mm. An empirical correlation was proposed for the mean bubble diameter prediction with a discrepancy of less than 25% from the measurements.