You'll be wanting the 18-18 1/2, which I think is the current XL. According to History Preservation, BR revised their shirt sizing a while back so that everything is effectively one size smaller. 18 equates to a FW 17 (I have a Neal chambray for comparison, as well as a few other designs). Perhaps intentionally, the work shirt sleeves are quite short – the cuffs will not entirely cover a watch, for example, or protrude from a jacket sleeve. I generally wear them rolled up. Also, they're prone to coming unbuttoned at points of stress – the buttonholes seem to stretch more on these shirts than the average chambray. Having said that, the character of the chambray is more "three dimensional" than just about any other work shirt I own (FW, Papa Nui, Homestead, Orslow, Sassafras, probably some others I've forgotten). The Gibson shirts may well have slightly different measurements.