Review on Solid Short Sleeve Marquis Light by Athenasby Whitney

The shirt itself is good but the website only lists the sizes "Medium, Large, Extra Large". It says what they mean in terms of several measurements but not the one I know to buy - neck size. If you only know your neck measurement is 15.5, what shirt size should you buy? I misjudged the size. It was close enough to wear on those days when I don't need to look my best but needed to go down a size.