A device and method for reducing a size of feed material. The device has a knife ring that is stationary within a housing or rotates around an axis. The knife ring has rod-shaped knife carriers that are disposed concentrically around the axis in an axis-parallel position at a mutual circumferential distance from each other and that form an inner circumferential surface of the knife ring with their sides facing the axis. Each knife carrier having a receptacle for fastening a flaking knife. The flaking knife protruding by its edge between two knife carriers into the flaking chamber. A measuring instrument includes at least one distance sensor that measures a distance of the inner circumferential surface and/or the edges of the flaking knives from the at least one distance sensor.