身躯 is a formal, literary word for the whole physical body, often emphasizing size, build, or strength (e.g. an ox's body, an athlete's frame). It is less common in everyday speech than 身子 (colloquial) and more specific than 身 (a general noun/classifier). Unlike 身材 (body shape/figure) or 人体 (human body as an anatomical object), 身躯 carries a sense of scale or presence.