I have posted this question for Gryffin, who I know owns the blazer in question, but please feel free to chime in if you have it as well/or have insights on it's sizing.
The blazer in question, Helmut Lang Smoking Tux, has been around for years. It is no longer being sold in retail, but can be found on various resale sites. I have been wanting it forever, and still do, but somehow never got around to getting it. Well, I have just randomly found one, on an actual online store site, so it is new, which is a huge plus for me. There is only one, and it is size 6.
Currently, most of my blazers that fit well are size 4. This includes a couple of Veronica Beard jackets, including some Millers, which are all 4. I have a recent Smythe in size 6, that fits well, but Smythe tends to run small. My one and only Helmut Lang (in Finds), which is a bit of a moto/blazer hybrid, is also a 4 and fits well. I prefer my blazers properly tailored/fitted, especially in the waist area, but am happy to settle for a slightly (but not very) fluid fit of something I have wanted this long.
Thus I am very curious to hear about the sizing of this particular jacket, and if 6 might be a possibility for me.