The stock plans are offered in 3 different size versions. The length of the building is unimportant as you could theoretically make the building as long as you want, provided you took into account the curvature of the earth. It's a simple concept. Where the math comes into play is the width of the building. The arches are really just segments of a circle, I forget my geometry but I believe it is called an arc. Just a segment of the circumference of the circle.
The layout for the jig to create the arcs which will become your arched trusses are made by fixing your tape measure at a specified distance away then swinging the other end through a segment of that arc, marking with a pencil as you go. I ordered the 20 foot wide design and it called for an approximately 14' arch. I wanted more height as the Diesel Duck will be close to 15 feet from the bottom of the keel to the top of the pilot house so I went with approximately 15 feet. When installed on the 20" knee wall I should be close to 16 feet. The building of the jig will require at least 1 sheet of plywood, more likely 2.
In my driveway I roughly marked out what a 15' arc would look like to know how much plywood I would need. I found that the widest part of the arc would fit on a 2' wide by 8' long sheet of plywood. So I bought my 1/2" plywood and ripped them in half resulting in 2 pieces of 2' by 8' strips of plywood. The second sheet I cross cut for a 2' by 4' strip. All together giving me 20' feet of space to construct my arch. Working in my shop using a couple of work tables I affixed a nail in one table and at the second table, approximately 15' away I drew a few layout lines perfectly perpendicular to the imaginary line from the distant nail. I then lined up a section of my ripped plywood carefully on the layout lines. I hooked a tape measure on the nail and pulled the tape to the predetermined measurements calculated from the instructions and swung my arcs out on the plywood using a pencil carefully held at the appropriate measurement. I repeated this process for all the required marks on the other two sheets of plywood and the layout was done.
All that is left is to tie a string to the nail and pull it tight to each side of the strips of plywood to make your marks for the appropriate angle to cut the plywood to. This angle allows you to connect all sections forming the jig, which is the arc of a 15' circle. I cut the appropriate angle, attached the sections together with gussets and glue on the underside of the jig and installed the clamping blocks. All the specifics are in the shed construction plans.
I brought the jig back to the shop and cut the aforementioned angles on the jig. It was now time to cut some 2x4's. Ideally it would be great to be able to go to the lumber yard and carefully pick out all your stock. I needed 72 10' 2x4's and I simply ordered them through my lumber supplier. I was very happy with the 10' stock I received, but I learned it was not perfect. For things to go exactly as planned you would need perfectly defect free stock. I mean completely defect free! No knots or twisted grain, perfect. As you cut your 2 by stock into 3/4" strips any defect in the wood will create a weak spot that will simply break when you begin to wrap it around the jig. I found this not to be a huge problem as the construction process requires using smaller pieces so butt joints are not an issue and will not weaken the structure. I just did the best I could, generally yielding 2 or 3 complete 10 foot sections out of a 2x4. I will still be able to use the smaller pieces that came from the ripping process.
I also ripped and cut to size all the 6 inch blocks required for each arch, nearly 400 in all. Additionally I ripped and cut to size the 24 inch sill and ridge blocks that are required. I used the jig and some tracing paper to transfer the portion of the arc that the 24" blocks would cover to the 24" blocks themselves. I then cut those slightly curving marks on the band saw. You could use a handheld jigsaw if you had to but the band saw greatly speeds the process up.