If you’re on a mission to find fitted shirts, you’ve come to the right place. A lot of guys (along with their moms and girlfriends) don’t actually know how a dress shirt should fit.
The seam where the sleeve attaches to the body of a fitted dress shirt should hit close to the top of your shoulders. Mass brands tend to cut their shirts too large (to fit more people, of course), and so you’ll often see the seam sliding down onto the arm.
Collar size is measured in inches from buttonhole to button when laid flat. You should be able to button the collar easily with room for two fingers to fit—any more and it’s too loose, any less and it’s too tight.