I have spent a lot of time reading while I have been laid up sick. There is now reason why you could not go several feet larger on your tear than the actual trailer. According to my "How To Build Trailers" book, many people use and unmodified model T frame which was only about 3.5 feet wide and just built a wooden frame on top of it. Many of these unit are still rolling around! I have a heavy duty 4x8.5 foot trailer and I am planning a 5x10 foot tear for it. According to everything I read in the books there is no reason why that should not be solid as heck! There are even plans for a 100% wooden trailer in my book. Heck 2 and 3 story house are made entirely of wood! Having all that steel to support a 1000 pound tear is just overkill. My floor will be a 1" thick frame sandwiched between 2 pieces of 1/2 plywood. When I gave the info to my brother (and engineering student) he told me that would support up to 1500 pounds just on a two foot overhang.
Anyway just my research and my opinion.