A trailer-detection system (10) includes a radar-sensor (16), a camera (26), and a controller (40). The radar-sensor (16) is used to determine a range (18), and an azimuth-angle (20), of a radar-signal (22) reflected by a feature (24) of a trailer (14) towed by a host-vehicle (12). The camera (26) is used to capture an image (28) of the trailer (14). The controller (40) is in communication with the radar-sensor (16) and the camera (26). The controller (40) is configured to determine a position in the image of a viewable-feature (24A) of the trailer (14), determine a trailer-width (34) and a trailer-height (36) of the trailer (14) based on the position and a range (18) and azimuth-angle (20) to the viewable-feature (24A) indicated by the radar-sensor (16), and optionally determine the trailer-length (32) based on the range (18) and azimuth-angle (20) of the radar-signal (22) reflected by a hidden-feature (24B).