If you're contemplating small versus x-small, or some such sizing quandry, it's difficult to say . . . . I own a couple of their jackets and buttondowns as XS and they fit perfectly, whereas I'm quite pleased with the S fit on the hoody (in earlier days, they had "slim fit" and "classic fit" for hoodies, the classic fit being bulky and un-fly, but now there's not these two categories and the fit is generally quite slim-ish); and for the thermals I cannot imagine even your slimmest, most pint-sized dude would be comfortable in XS of these, as the pit-to-pit is like 16.5 inches or however many centimeters . . . . So I'd wait before making a call, wait till Context, Haven, or the two above sites (Roden Gray and Four Horsemen) toss out measurements.