noun
large build; broad-shouldered physique
Typically describes a middle-aged man's imposing, sturdy frame. Often carries a nuance of dignity or prosperity.
彼は恰幅のいい体格をしている。
He has a large, imposing build.
恰幅のいい中年男性が店に入ってきた。
A broad-shouldered middle-aged man came into the shop.
体格 is a general term for physique or build, while 恰幅 specifically implies a large, sturdy, often middle-aged frame with a sense of presence.
The derivation is uncertain. The kanji 恰 (just like, as if) and 幅 (width, breadth) are conventionally associated with the meaning of a broad, imposing physique.