Pretty sure a US small = JPN medium (ie. one up for every size)
Someone correct me if I'm wrong
Edit: Well I just compared sizing on the flannel shirts and it seems like the overall length and sleeve length are a couple of centimeters shorter on JPN sizing. Never realized that. Kind of ruins my plan to proxy certain items from Japan.
@OccultaVexillum did you go true to size on the soft twill shirts you picked up in Australia? Your photos made them look not too...