A bale size sensing mechanism (80) for an agricultural round baler (10) having a bale chamber (20) partially defined by continuous side-by-side belts disposed between a pair of spaced-apart side walls (19) and trained around a plurality of rolls. Bale size sensing mechanisms (80) disposed in at least one of the side walls detects the density of the transverse ends of a bale forming within the chamber and provides the measure to a controller which determines the transverse uniformity of the bale. Each sensing mechanism includes a generally planar element (90) having a wedge-shaped periphery with a vertex, the planar element being pivotally coupled to the baler (10) adjacent to the vertex and a portion of the wedge-shaped periphery projecting into the bale-forming chamber to interact with the bale forming therein. The wedge-shaped periphery minimizes open areas within the bale chamber into which turbulent crop could enter and jam the sensing mechanism.