Beside payload size, what precise factors are considered to choose the fuselages shape? I am currently reading this very interesting book, which explains -I guess- most of the things you need to know in order to design aircraft.
Yet it is not clear to me besides the payload size what other factors are precisely used in order to determine the exact shape and size of the fuselage. I know that aerodynamics are considered, but that's very broad...

I read that the conic lofting method is usually used to determine the exact shape of the fuselage. But I would need some more explanation to get a better understanding of how this is determined.
In other words: I know my wing size, airfoil, payload size and would like to design the fuselage. How should I proceed?

Thanks

EDIT: Feel free to share the pragmatic approach

What bothers me is that I ended up finding a webpage with some interesting guidelines, but contains many constant values I have no I idea where they cone from: http://www.barnardmicrosystems.com/UAV/uav_design/guidelines.html

eg of such values: wingspan: 1.041, 0.382; step 3 length: 1.775
Assuming an engineering background, here’s a short list of factors affecting fuselage shape and size. 


aerodynamic considerations
volume considerations
structural and elastic considerations
operational and interface considerations.


Aerodynamically, drag and stability are the major concerns.

Volume wise, payloads, subsystems, cargo, propulsion, etc must be considered. So it might take a few iterations for a preliminary design to converge at a feasible and optimum solution. 

Structurally, elasticity of the fuselage is important, at least for weight efficiency, and general comfort. a bad shape can cause unwanted structural modes, which could risk failure in flight, or may cause difficulty in controls. If you were ever on a 747, you probably noticed how much it snakes towards the rear of the plane. The oval shape of the fuselage has parts to blame.

Finally, sometimes doors, openings, and structural interfaces dictate geometries of fuselages.

Additionally, geometric constraints such as tail strike prevention, or belly landing can be other reasons to shape the fuselage.