pixelate – 15 1/2 is not mainly the neck size; it is only the neck size, measured in inches from end to end. I know that.
Obviously, the rest of the shirt is made in proportion to the neck size.
People who like slim-fitting shirts (read: people who don’t want their shirts to look like garbage bags) are better off going down a size than a fit when it comes to Express. The reason is that the fit is mainly affecting the lower end of the shirts, and the size affects the shoulders and chest, which is where the nicer fit will show.
If you don’t close your top button (no one does), a size down will not make you choke. You’ll barely notice it.