Here are some pics that may (or may not) help prospective buyers of this jacket.
I'm 6'1, 185lbs and take a 35/36 dress shirt sleeve length as I have longer arms. This is the size 42 (XL) jacket.
Most leather jackets don't work for me as I need a size large for the body, but the sleeves are too short. Moving up to an XL makes the body to big and the sleeves are usually still too short.
As you can see here, the 42 (XL) sleeve length is great for me. The jacket is still too stiff to try and button up, but I can do it and it's definitely a slim fit across the chest. Chances are I'll never button it up anyways. I would compare this fit to a Schott 42 long.
Another reference point is I wear a size 42 in blazers from brands like Canali, Boglioli, Tagliatore and Lardini for the body and then get the sleeves lengthened slightly.
I usually take a size large in every other brand of jacket, t-shirt, sweater etc that I buy from various brands.
I tried on an XXL (44) of the same style of jacket in the split steer hide from IH at the store but it was too big in the shoulders, big in the chest and the arms around the biceps were baggy. This could have been because of the softer material or the size. I am interested in the Steer hide and will probably get the same 42 (XL) size in that as well if I find one.