The opening in the face frame is 5 1/4" high x 23 7/8" wide (133.35mm x 606.43mm). You'll have to allow for the door catch if there is one on your unit. The cavity is wider once you get past the face frame but the dimensions given are the size of an imaginary box that would fit through the opening without a door catch in place.
2006 23B Hybrid with 10k round bar WDH
2011 F150 4x4 SCREW Ecoboost, Max Tow, Integrated TBC, 3.73 LS axle, Firestone Ride Rite airbags.