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.