Bubble size is an important variable in aerated stirred tanks as it determines the surface area available for reactions. In the bubble break-up process, impellers play a key role. Despite this importance, research into the effect of impeller design on bubble size is scarce. In this work we study the effect of two impellers, with and without a stator, as well as the effect of airflow rate, impeller speed and surfactant concentration on bubble size. Results show that there is a critical impeller speed above which bubble size is not further decreased, regardless of the airflow. Operating at this critical speed results in the smallest bubble size possible without additional turbulence. The reduction in bubble size caused by a stator was quantified for the first time and, interestingly, it was found that the stator also reduced the critical coalescence concentration. The implications of these findings for the design, evaluation and optimisation of impellers are discussed. (C) 2020 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.