His neck is 18.5 inches. I mathified this (C=2pi*r) and figured out the radius of the circle that is his neck and then cut out a square that could bound the circle of his neck (each side of the square was 2r). Turns out this is TOTALLY the wrong thing to do. This was me, the modern day educated woman, applying math to a non-math problem. By doing it this way I ended up with a VERY loose neck in fact so loose he could pull the tunic on over his head without any slit opening (a medieval Norse woman would be -scandalized-). I ended up sealing the slit on the neck of the fighting tunic and he just has a nice faced square neck opening on it.
BUT this did allow me to realize the error of my ways BEFORE I tried to make the Viborg shirt for him. Instead of making the opening a square that could contain the circle of his neck I need to remember that the square on the shirt is made of fabric.. so it will deform to fit his neck. So really I need to figure out a square that has the same total exterior measurements as the circle of his neck. In this case it means that each side of the square needs to be 4.625″ long (18.5/4). I will probably fudge that to 4.75 inches just to give him a smidge of breathing room.