Before I forget - quick tip: these jackets are VERY TIGHT as they are a functional jacket, built to work as body armor for motorbike use and for that purpose, they can't be sliding around the body.
I am six foot one inch and measure exactly 41 inches around the chest and I do not have particularly broad shoulders. I got a size 42 regular and it fits like a glove. There is no room for a denim jacket underneath, only a shirt and a t-shirt. These things are windproof though, and unless you are going to the South Pole, you won't need more than 2 layers.
No fake weathering on my jacket too by the way, a touch of Pecards Gel on the amber stripes to waterproof them, and otherwise, just lots of wear and use