The air entrainment due to a plunging free-surface flow in the Inverted-Shroud gravitational gas-liquid separator (IS-separator) is studied in order to determine bubble size. This kind of flow system is an unconventional one. After air entrainment happens, the continuous liquid phase tries to carry the generated bubbles downstream into the IS-separator. We describe the equipment features and outline that its performance depends on the size of the generated bubbles formed during the air entrainment process. It is also shown that bubble diameter can be expressed as a function of the free-surface velocity and a characteristic length Ldis, where the kinetic energy of plunging free-surface flow is dissipated. Some Ldis correlations from literature are used to determinate bubble diameter as a function of flow conditions, fluid properties and geometry of the IS-separator. The predictions are compared with reported experimental results showing good agreement. On the other hand, it is shown that bubble diameter predictions using classical models are not suitable. The results suggest that the models based on Ldis are appropriate, however more comparison are needed in order to get conclusive results.