To be extremely safe, get a large.
MEDIUM
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about 720 mm × length of about 480 mm × width about shoulder width of about 430 mm × sleeve length of about 230 mm
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US MEDIUM
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20 in. width x 29 in. length, in mm it's 508 mm width x 790mm length
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Though the Japanese medium is really a little bit bigger than an American small. Though I wonder how accurate these online measurements are. Perhaps you could ask people what size they have? :p