How Your Shirt Should Fit

Seams should rest at the edge of your shoulders. Not on top, and not over them.
The fit on your torso is a matter of preference but it should fit comfortably around the chest, under your armpits and across the upper back.
The buttons on your chest should not be pulling at the fabric.
Your collar when buttoned should leave room enough for one finger to fit comfortably between your neck and the collar.
Sleeves can be close fitting, but you should still be able to bend your arm without feeling the fabric stretch.
Check where your sleeves end when you let your arms hang by your sides. They should end right at the base of your thumb.
If you plan on wearing the shirt untucked, raise your arms and make sure none of your belly is exposed.