I just read {glanced, really} through this entire thread, and I'm surprised there's no mention of shirts.
I'm not a "big" guy by any means 5'-11", 185+/- lbs. I have a harder time fitting shirts than jeans. I prefer a more fitted, tailored fit for my clothes, even for denim-wear (which is all I ever wear). I really don't like boxy, baggy, over-sized clothes anymore. Work shirts are usually too boxy in the mid-section and waist, and why I prefer westerns for the slimmer body.
What I never see mentioned in any discussions of shirts of any type (tees or L/S button front) is the proportion of shoulder-to-chest measurements. I find that most shirts' shoulders at the top of the sleeve seam are too narrow compared to the chest width at the arm pit. Extra shoulder width would do a lot for comfort and ease of motion without changing the body width.
The best fitting shirts I have are The Flat Head westerns, in XL/44. The are cut like they are tailored specifically for me. Wide in the shoulders and chest, narrow in the waist, perfect length body and sleeves. If anything, the arms are a bit tight. It seems they don't work for most western guys though. Kyle, on SuFu, used to work for TFH in Japan some years ago, and he said Masa-san develops patterns cut for his own body, which even for a small Japanese man he has a large shoulder-to-waist ratio. Then the pattern is scaled up and down for other sizes. (Correct me if I'm wrong). However TFH does it, it works for me.
The other shirts I've found that fit me well are Wranglers Retro and Western Cowboy Work cut. They're super cheap - in price and quality - but they fit me well. A L gives me more room in the shoulders and chest, but are too long in the body, so I wear a M.
I wear an XL in IH westerns, and they are not ideal for me. I LOVE the styling, fabrics, details, etc. The chest and shoulders are tight, too wide in waist width, the body sometimes too long. Not including the collar button, the top 2 snaps pull outwards and the fabric looks distorted. Dean DelRay mention this too in his podcast interview w/ Giles. I need an XXL in the chest and shoulders, and a L everywhere else. I don't have an IH Work shirt, because the measurements look too wide in the waist and opening for me.
First world problems, I know, LOL! 