Awesome man! The links from "sakujapan" (a site I never heard of before) were really cool. It seems like you're talking about suits, but are you looking for a more professional wardrobe or more casual? For suits, I see you seem to appreciate the sleekness and shininess of the fabric, and the overall cut is slimmer because Asians are built smaller (speaking as one). Of course, especially since you're 27 you can break up the suit as you wish. If you're looking to build a casual wardrobe, there are plenty of sources to admire Japanese streetwear. You can start with gq and just type "japanese streetwear" into the search bar and you'll get to see some of the cool ways they dress up over there and maybe start to look at some key pieces a hip japanese dude might rock- varsity jackets, leather jackets, raw super slim denim, heritage sneakers, sometimes bowler hats or fedoras, etc. You can look at the designs of Comme des Garcon, Yohji Yamamoto, and a Bathing Ape. You can score these brands online, particularly at Mr Porter. Since japanese streetwear is largely about swagging up American classic pieces, I think you can find most of what you're looking for pretty easily if you pay close attention to fit, style, and color (Asian fashion is brighter, whereas current trends, though colorful, are more faded and pastille). You can look at the styles of Kid Cudi, Pharell Williams, and some Kanye West (but take Kanye with a grain of salt lol). Best of luck man! Actually, if you can, post up some pics or something of clothes you acquire or whatever, I'd be really interested to know what you come up with!
^^^ I just typed all of that out before realizing that you were specifically looking to address sizing lol. You're probably gonna have a tough time. Trial and error, nothing else. Every brand is different. If you can, seek out certain stores that carry particularly chic asian inspired brands and just try stuff on. Sorry, I really can't think of any better advice.