How the dress shirt collar should fit
The dress shirt collar should fit comfortably without being too loose. As tight or snug as possible without being uncomfortable. You should be able to fit 1-2 fingers between the collar and your neck when it is buttoned. You should not have a large gap showing between the collar and neck. Heavier men should make sure their necks are not puffing out over the top of the collar.
Shrinkage
Proper Cloth adds a standard adjustment of 0.4” to the Collar Around dimension to match industry standards. Depending on the fabric type, additional counter-shrinkage adjustments may be applied.
If you never wear a tie
If you never or rarely wear a tie, having the collar a bit tight when buttoned is totally ok and in some cases will make the open collar stand up more nicely.
If there’s a gap behind your neck
If the collar feels uncomfortably tight when it is buttoned, but you can still see a large gap between the back of your neck and the back of the collar band, then you will want to consider using a neck forward posture adjustment.