A trailer-detection system configured to determine a dimension of a trailer towed by a host-vehicle, said system comprising: a camera mounted on the host-vehicle and configured to capture an image of the trailer, features of the trailer that are in a field-of-view of the camera being defined as viewable-features, the viewable-features including a right vertical edge, a left vertical edge, a top horizontal edge, and a bottom horizontal edge; a radar-sensor mounted on the host-vehicle and configured to: determine ranges and azimuth-angles of a radar-signal reflected by the viewable-features and one or more hidden-features of the trailer that are not in the field-of-view of the camera; and a controller in communication with the radar-sensor and the camera, and configured to: determine a trailer-width of the trailer based on a position in the image of the right vertical edge, a position in the image of the left vertical edge, and a distance to a vertical plane comprising the right vertical edge and the left vertical edge determined from the reflected radar-signal; determine a trailer-height of the trailer based on a position in the image of the top horizontal edge, a position in the image of the bottom horizontal edge, and a distance to a vertical plane comprising the top horizontal edge and the bottom horizontal edge determined from the reflected radar-signal; and determine a trailer-length as a longitudinal distance determined based on a range and an azimuth-angle to a most-distant hidden-feature that is closer to the host-vehicle than a maximum trailer-length. a camera mounted on the host-vehicle and configured to capture an image of the trailer, features of the trailer that are in a field-of-view of the camera being defined as viewable-features, the viewable-features including a right vertical edge, a left vertical edge, a top horizontal edge, and a bottom horizontal edge; a radar-sensor mounted on the host-vehicle and configured to: determine ranges and azimuth-angles of a radar-signal reflected by the viewable-features and one or more hidden-features of the trailer that are not in the field-of-view of the camera; and a controller in communication with the radar-sensor and the camera, and configured to: determine a trailer-width of the trailer based on a position in the image of the right vertical edge, a position in the image of the left vertical edge, and a distance to a vertical plane comprising the right vertical edge and the left vertical edge determined from the reflected radar-signal; determine a trailer-height of the trailer based on a position in the image of the top horizontal edge, a position in the image of the bottom horizontal edge, and a distance to a vertical plane comprising the top horizontal edge and the bottom horizontal edge determined from the reflected radar-signal; and determine a trailer-length as a longitudinal distance determined based on a range and an azimuth-angle to a most-distant hidden-feature that is closer to the host-vehicle than a maximum trailer-length. determine ranges and azimuth-angles of a radar-signal reflected by the viewable-features and one or more hidden-features of the trailer that are not in the field-of-view of the camera; and a controller in communication with the radar-sensor and the camera, and configured to: determine a trailer-width of the trailer based on a position in the image of the right vertical edge, a position in the image of the left vertical edge, and a distance to a vertical plane comprising the right vertical edge and the left vertical edge determined from the reflected radar-signal; determine a trailer-height of the trailer based on a position in the image of the top horizontal edge, a position in the image of the bottom horizontal edge, and a distance to a vertical plane comprising the top horizontal edge and the bottom horizontal edge determined from the reflected radar-signal; and determine a trailer-length as a longitudinal distance determined based on a range and an azimuth-angle to a most-distant hidden-feature that is closer to the host-vehicle than a maximum trailer-length. determine a trailer-width of the trailer based on a position in the image of the right vertical edge, a position in the image of the left vertical edge, and a distance to a vertical plane comprising the right vertical edge and the left vertical edge determined from the reflected radar-signal; determine a trailer-height of the trailer based on a position in the image of the top horizontal edge, a position in the image of the bottom horizontal edge, and a distance to a vertical plane comprising the top horizontal edge and the bottom horizontal edge determined from the reflected radar-signal; and determine a trailer-length as a longitudinal distance determined based on a range and an azimuth-angle to a most-distant hidden-feature that is closer to the host-vehicle than a maximum trailer-length.