Our shirts are sized according to neck size. Choose the neck size you ordinarily buy. For the separate collar that is worn with a tunic shirt, choose half an inch larger than the shirt. This is because the separate collar has to go on the outside of the collar on the tunic shirt. For example, if you wear a 16 inch ordinary shirt, you wear a 16 inch tunic shirt but with a 16 1/2 inch separate collar.