A belt assembly includes a belt retractor, a sensor having a detection field, and a belt retractably payable from the belt retractor through the detection field. The belt has a webbing and a plurality of markers fixed to the webbing. The webbing has a longitudinal length and the markers are equally spaced along the longitudinal length of the webbing. The sensor senses the length of the webbing paid out. A size of the occupant may be determined based on the length of the webbing paid out, and the size of the occupant may be used to classify the occupant in classes.