Abstract: A size sensing X-ray cassette tray having a pair of longitudinally spaced opposing front and back cassette clamps linked together by a center pivoted arm and link members for unison movement. A side sensing arm is connected by an elongatable link member to a pivoted leg which is actuated by a probe of an X-ray system. Arcuate potentiometers are coupled to the pivoted arm and the side sensing arm so as to provide electric signals corresponding to the x and y dimensions of a mounted cassette. The potentiometers are nonlinear so as to provide respective cosine and sine correction and thereby make the output signals substantially linear with the dimensions of sensed cassettes.