umm, you can buy lumber pretty much any even number length you want, 8-10-12, etc up to about 18 to 20 feet, after that it's special order.
the most COMMON length builders use for plans and planning material purchases is 8', because that's the side of most sheeting, plywood, panelling, etc.
so in the thoughtline you were going with, most builders will plan out something taking in consideration 8 feet. for example, an 8 x 12 coop. This would take 3 sheets of siding or plywood for the long side, two for the ends. The studs for the walls are even already precut at 92 5/8" to allow for the wall framing plates and sills, so they'll come out at just the right height to make the wall 8 feet tall for the sheeting.
the good thing about doing something 12' is that there is a lot less cutting, because the math works out so well for 8' long or 4' wide