Costume national.

I never tried any Dior Homme pieces, so I can't compare. I do have one Costume National shirt though; it is a size 40, whereas I usually take a 39 (40's not a big step up, but you get the idea). The shirt is the tightest one I own, mainly because the armholes are cut pretty high; had the sleeves not been on the long side I think I could have gone up to a 41 and still had a fairly form-fitting shirt; a 39 almost surely would not have fit me. I wish I could give you a more general example, but to summarize for shirts at least, Costume cuts for the very slim and lanky. I'd say go one size up for a close fit, two sizes up if you want a little more room. I have no experience with pants/jackets/sweaters, but a size up might be a better idea with those too.

Pretty slim. I wouldn't go a size up though, unless it is a physical impossibility to get yourself into the article. I did that with a shirt once, and it was a mistake. It never looked quite right. Costume does cut very slim, with very high arm holes and fairly narrow shoulders - though not as narrow as Dior's. But the shirts are designed to be worn that way. It's like a fine wine. It's hard to get used to at the beginning, but once you do, you can never go back.

I have had some strange experiences with Costume National (a brand I really like). I actually find their clothing to be fairly on the big side. When I would try on their clothing I assumed it would be like Helmut Lang and I would have to size up to a 32 waist pant,but when I did, they were so large, even the 30 fit well and not tight. I have found the same for their jackets, normally I can wear a 40R in American sizing, but I find for Costume I can take a 38R. I own 2 pieces of their outerwear and 2 pairs of pants.