I am soon starting my SOF and have been thinking/considering the same issue. I have Morris and Starr's books (with Cunningham's on order). Likely I will use a modified Morris approach.

Anyway, I am currently paddling an S&G (protoype by someone I bought from) that is 17.5' length and 20" beam with a foredeck height of 9" and aft deck height of 7". It's lower volume than the production boats I have but it's still higher in volume than what I want to build. I have a freeboard of about 3". What I figured out is that Morris' approach to determining width would actually leave me with a boat at 20.5" in beam. The deck heights from Morris method (fore and aft) would be lower, however, with aft deck height being about 4.5". The length I haven't determine yet. Bottom line, based on my experience with the S&G, I know I can squeeze probably pretty comfortably into a beam of about 15-16" but it may be too tender for me yet. So, I am going for a beam of about 18". The length I may add several inches (to be on the safe side).

The way I look it, the first SOF will be a protype followed by a more suited model based on usage later.

sing