My mother used to remark "MIND THE GAP" whenever we stepped off the subway train. She related it with some mix of French and German dialect which cannot be duplicated. It was a call to caution to avoid the space between the train and the platform, the heeding of which comment would set us on the right path to long, successful lives.
So today, I can offer our gentlemen the same assurance of long and happy lives if they will simply mind the gap. An ill fitting shirt is the mark of an amateur, someone who will certainly care less about a client's business, investments, healthcare, (fill in the blank) than those whose grooming shows care.
A dress shirt should fit well everywhere but especially around the neck, near the face (presumably the focal point for social intercourse.) The collar should fit snugly and smoothly against the skin with room to insert two fingers between the collar and neck. This allows the tie to fit well and fall properly. So gentlemen, best foot forward please. MIND THE GAP!