We study experimentally the maximum thickness of the flowing zone in a rotating drum of diameter D half filled with glass beads of diameter d. Several drum diameters and beads sizes were studied, bringing a special attention to geometrically similar systems, i.e. of constant ratio D/d. The rotation speed ranges from 2 to 20 rpm, the minimum is chosen such that the system is in the continuous flow regime. The study of the flowing zone thickness versus the diameter of the beads and the diameter of the drum shows that the ratio D/d is the relevant parameter.