It's not really a piece that can be measured easily given its open and belted style. I'm a jacket size 42, and while I'm generally a large in various sportswear, in Eidos knitwear (which I also find big), I'm a M. I took a M in the Aran for a fitted look in the shoulders and chest, so the fronts don't really stay overlapped. I could have done large, but it would have looked slouchier than I wanted. If you're a 40, the M may be in the range and work for you as well, with a bit of slouch. If you're a 38, I'd say take the S.
Alternatively, order two sizes then return the one that doesn't work. It's worth the return shipping to make sure you get the size you want, assuming it's not cost prohibitive.