It runs huge, you should go down 2 sizes of your regular SISP sizing. I had the stealth parka in M, and it was still too big for my taste. Fit around the shoulders and chest was ok but the jacket billows out towards the bottom so the side-on silhouette is wack. For reference, for regular SI and SISP I wear an XL, am 83kg 178cm and muscular with wide shoulders, chest and ass. Wear a Medium in Acronym for regular fit, Large for profile fit. Measurement wise, anything below a roughly 60cm across chest measurement is tight (wearable, but slightly restrictive).
It's a super cool jacket though, returned it cause of the fit but if you can get one and you like the fit definetely worth keeping.