Wastewater treatment by dissolved air flotation (DAF) relies on the adhesion of particles or flocs to air bubbles, formed when pressurized and air-saturated water is released in the DAF reactor. Bubbles sizes are of paramount importance. A differential gravimetric bubble separation method was used for the estimation of the bubble size distribution. The relation between the air bubble diameter after water depressurization and the pressure of the air-saturated water, its temperature, and the ratio of air saturation was studied. The bubble size distribution was proved to depend on the pressure of the air-saturated water. The temperature of water and its percentage of air saturation did not have a considerable effect. The experimental results and their treatment define the bubble size distribution, which is a dominant factor for highly efficient DAF-flotation.