Start with your usual dress-shirt size. Unlike many casual shirts that use only small, medium, and large sizing, these styles use dress-shirt sizing by collar measurement, which gives you a more precise fit than standard S, M, and L sizing alone. That helps create a sharper shape through the collar and body, which is especially useful if you plan to wear your shirt untucked under a sport coat or tucked in at the office.
If you are between sizes or prefer a little more room through the chest and waist, check the Fit Guide before choosing.