In order to fit comfortably, a shirt’s collar size should be determined by fit and not by measurement. Shirt makers are supposed to allow for about a half inch of shrinkage but some manufacturers provide much less leeway which means that a perfectly fitting new shirt will end up choking the wearer after several washes. To confirm that there is room for shrinkage in a new shirt, try it on and make sure you can easily slip two fingers between your neck and the collar. Another method is to lay out the shirt and actually measure the distance from the center of the button to the outer edge of the button hole to make sure it is half an inch more than your actual neck size.
If wearing a turndown collar that is semi-spread or spread style, the points of the collar should end beneath the jacket. The collar should also remain flat against the body no matter how far the head is turned.
The majority of ready-to-wear shirts are made to fit obese men. As a result, everyone else has to put up with a sea of excess fabric or pay a tailor to alter the shirt. The most basic alteration involves taking in the shirt along the side seams of the body and the arms. For a truly form fitting garment, two darts will have to be added in the back. While the feminine aesthetics of darted shirts are a matter of debate, this is a moot point with formal wear because it is not good form to remove one’s jacket at a black-tie event. What’s important is that the less excess fabric there is, the smoother the shirt will lie against the body and the neater the overall outfit will appear.
The shirt’s shoulder seam should sit on top of the curve of the natural shoulder, not down the side of the upper arm.
The shirt’s sleeves should be just long enough that they don’t pull back from the wrist when the wearer extends his arms fully when wearing a jacket (the jacket’s armhole will impact the practical length of the shirt sleeve). Because mainstream shirt-makers save money by offering shirts only in odd numbered sleeve lengths half of all men will likely end up with a sleeve that is too long and subsequently too blousy. (A so-called “34/35” sleeve is really a 35 – it can’t be both.) This excess fabric can bunch up within a narrow jacket sleeve causing it to pull back the shirt sleeve when the arm is extended. Fortunately, a good tailor or dry cleaner seamstress will be able to shorten a shirt’s sleeves if needed.
In order to stay put at the wrist, sleeve cuffs – both French (double) and single – should button snugly. If your hand can slide through a fastened cuff then it is too loose and the buttons or link holes need to be adjusted.
A French cuff’s bulk should also be able to fit easily inside your jacket sleeve to allow the latter to move independently of the shirt sleeve for reasons explained above. If it doesn’t then find another shirt.