It's the blue link right next to the word SIZE when you're looking at the coats or pants. The easiest thing to do is go based on what BDU size you are. If you're a medium regular jacket and that fits well, then you're chest is probably 37 to 41 inches, according to the chart. That means you'll want the ABU coat either 38R or 40R. You just have to take an educated guess as to which of the 2 sizes, based on how your current MR sized coat fits you.
As an aside, I think ABU looks pretty cool, and I want to see how effective it might be out here. It's a little hard to tell from pictures, but I wouldn't be very surprised if it worked well here.
