The parts make the gun. That's my experience. I start with a concept of what I want to build, but find halfway through the parts are telling me I have to change course.
Barrel and lock, mentioned above by Rich, dictate the breech size and the wood around it. Then the wrist, comb and butt are dictated by the buttplate. The underside line is determined by the guard, what it allows you to do stylistically, a step toe or a straight run. If you ignore the parts and forge ahead, sometimes you end up with an odd duck looking gun.
Tom