Somatotypes and frame size
Sheldon's somatotype model (1940s) classified people as ectomorphs (lean), mesomorphs (athletic) or endomorphs (rounder). Most people are blends.
Wrist-to-height ratio is a simple proxy for skeletal frame: ectomorphs have narrow wrists relative to height, endomorphs broad.
McCallum's wrist-derived ideal measurements (chest = 6.5 × wrist, etc.) are a classic bodybuilding target popularised in the 1960s.