... or build 4'-7" wide using 5ft stock sizes.
... or build 4'-7" wide using any of a number of techniques for "
stretching your plywood" in construction tips forum sticky.
Just remember, for a traditional build you want the roof skin to be the widest piece (then the walls step in under that, perhaps sitting on top of the floor, perhaps skirting past the frame if tire sidewall clearance allows and you want the trailer frame not to show; then the floor width).
In other words, you have to figure out the 'schedule' of construction materials and joint designs in order to decide how it will fit the trailer and what the ultimate panel width of the roof will be. This "
Wall to Floor Attachment" thread will explain a lot.
Edit: There is also the Steve Fredrick's method, where the roof skin sits down on a ledge inside of the wall top, making it easier to build from the inside out and achieving a nice tight headliner/ceiling joint to the wall.