A study of the small bubbles created by gas dispersion mechanisms at the rear of a ventilated cavity attached to a cylindrical bluff body is presented. A critical Weber number model, which uses a CFD simulation of the continuous-phase flow field to simulate the rate of energy dissipation at the rear of the cavity, is proposed to allow prediction of the Sauter mean bubble diameter. The model is tested against experimental measurements for variations in gas headspace feed pressure and surrounding liquid velocity, with predicted diameters generally within 20% of the measured values for a critical Weber number of 1.2. In agreement with previous studies, a similar cumulative size distribution was found for all tested conditions, with the ratio of Sauter mean diameter to maximum measured bubble size equal to 0.61.