A bale size sensing mechanism for an agricultural round baler having bale chamber partially defined by continuous side-by-side belts disposed between a pair of spaced-apart side walls and trained around a plurality of rolls. Bale size sensing mechanisms disposed in each side wall 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 having a wedge-shaped periphery with a vertex, the planar element being pivotally coupled to the baler 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.