I think the best size is in the 60-65% scale range. This allows for lower cost, lower power engines, and performance in the 150-200 mph range. Using composites cuts the weight drastically allowing for greater useful load. Using a WAR type as an example, they typically weigh 750#-800# empty. They are supposed to weigh about 620#. Using composites it should be possible to get to the 550#-600#empty weight. This will lower stall speed and enhance performance. Construction would have to use pre-molded carbon granite skins over a substructure or skins that are designed to carry the loads. The picture is a fuselage that is made from a mold and structurally very strong. Once the mold is made it is relatively simple to make copies.