In order to characterize bubbles in dense, heterogeneous bubbly flows such as those encountered in industrial bubble columns, a new measuring technique based on the spatial correlation of phase indicator functions is proposed. By analyzing its principle of operation, it is shown that the correlation coefficient decreases with the ratio of the distance between two optical probes to the bubble size. Hence for a known distance, the bubble size, and more precisely the Sauter mean horizontal diameter of bubbles is accessible. A dedicated sensor has been designed using conical mono-fiber optical probes. It has been compared to alternate measuring techniques in the complex bubbly flows produced in an I.D. 400 mm bubble column with gas superficial velocities from 3 to 35 cm/s. The performances of the correlation sensor happen to be quite satisfactory, with typical uncertainties about 15–20%, and even less (<10%) in absence of flow reversal. Similar performances are also obtained for conical mono-fiber optical probes when measuring the Sauter mean vertical diameter of bubbles.