A rotary baler having a variable size baling chamber for the formation of a cylindrical bale. The baler is configured to measure diameter and the shape of the bale. To measure the shape of the bale, the baler is provided with at least one sensor arranged to generate information regarding the diameter of a bale formed in the baling chamber. The sensor is a non-contacting sensor and is arranged to detect the position of the outer surface of the bale or the surface of an endless conveying element located about the bale.