A trailer-detection system includes a radar-sensor and a controller. The radar-sensor is used to determine a range, an azimuth-angle, and an elevation-angle of a radar-signal reflected by a trailer towed by a host-vehicle. The controller is in communication with the radar-sensor. The controller is configured to determine a size of the trailer towed by the host-vehicle based on the range, the azimuth-angle, and the elevation-angle of the radar-signal.