How To Know Your Shirt Size? Posted on by Georgina Maxwell 0 Is Size 15.5 shirt a small? Measurements. We measure our shirts the traditional way, using collar sizes, so you will see our shirts quoted in inches – 15″ (small), 15.5″ (medium), 16.5″ (large) and 17.5″ (extra large). Leave a Reply Cancel reply Your email address will not be published. Required fields are marked *Comment Name * Email * Website