With the exception of the Italians, who border on the fetishistic relative to the fit of their dress shirts, most men wear theirs too tight in the neck, too short in the sleeve, and too full around the wrist.
The explanation for this is relatively simple: successive washings shrink the collar size and sleeve length, while most manufacturers allow enough cuff width for a large Rolex-sized watch to drive through.
The collar
Unless its collar fits comfortably, the best dress shirt is useless. With the top button closed, two fingers should be able to slide comfortably between the neck and the collar of a new shirt. Most fine shirt makes add an extra half-inch to the stated collar size to allow for shrink age during the first several washings.
Should the collar of a new dress shirt fit to perfection when first tried on, return it or risk being strangled before too long.