The BBs is 68mm so I would not want to go over 2.25" (57mm) so that would be 5mm on each side for joints. The Boom tube is the one between BBs there will be 3 diagonal tubes like this:

However, the top tube on mine will have a sharper angle to allow the stoker to be short as in 7-year-old size.
Like this with the 3rd tube:


I am also planning on hemp for the joints to be as eco as possible. It seems to come in two sizes .5 and 1.0 mm which should I get?