Hi Terry, I guess I am a little bit confused. Building a stand-up trailer from scratch, is quiet a costly and time consuming project. You say that the width intended will be 6 ft 6 inches, yet your bed required is only 4 ft 6 inches. It doesn't add up. You would like to have 5 ft 9 inches of standing room, including the dropped floor, yet only a bed for someone 1 ft and 3 inches shorter. ??? The 11 feet long seems realistic, but have you considered buying a vintage trailer, of about the same size, that is in fairly good shape? You could alter it's interior to suit your needs, probably for much less money and less time involved.
I bought such a trailer, a 10ft long (interior) a 56 Kenskill, and have completely rebuilt the interior (which probably wasn't necessary), and now have a wonderful stand-up, waterproof, vintage trailer, complete with a full length 6 ft 4 inch bed at the rear, a new dinette, up front, that converts into a great bed, and has a hidden porta-potty under the newly built cabinets.
I know that the money involved for the initial investment and my added cost of the rebuild, was much cheaper than going through a ground-up construction, (custom frame, suspension, wheels and tires. floor and wall framing, windows, door, interior cabinetry, wall paneling, exterior aluminum siding, RV door and mouldings, roofing, tongue, hitch, wiring and lights as well as many other hiddens costs, etc).
Perhaps others will feel differently, but there is just something about restoring a 40 to 50 year old vintage trailer, that garners the attention of other enthusiasts, while you are enjoying it's comfort, and believe me, the time involved is no where near what a ground-up project requires. Just my two cents. Enjoy your build and be sure to take pics and post em.

Roly, TDer and vintage trailer nut.