In many tracking applications, the extent of the target object is neglected and it is assumed that the received measurements stem from a point source. However, modern sensors are able to supply several measurements from different scattering cen- ters on the target object due to their high-resolution capability. As a consequence, it becomes necessary to incorporate the target extent into the estimation procedure. This paper introduces a new method for tracking the smallest enclosing rectangle of an ex- tended object with an unknown shape. At each time step, a finite set of noisy position measurements that stem from arbitrary, unknown measurement sources on the target surface may be available. In contrast to common approaches, the presented approach does not have to make any statistical assumptions on the measurement sources.