Welcome to the forum Bill.
A 4' x 8' would be fine if you are camping alone, but a 5 footer would be a lot more comfortable for two. It also depends on what kind of trailer you plan to build on, a custom frame (which can be any width), or a HF red trailer (which is usually 4'). Mine is actually just over 4 1/2' wide, so it will accomodate a standard full sized mattress.