How is kitchen extract sized?

Conceptually, from the menu. The cooking the menu demands determines the heat, moisture and grease the hoods must capture, which drives extract volume — and every unit of air removed has to be replaced with tempered makeup air, or the kitchen pulls itself into negative pressure. Char-grills and solid-fuel lines sit at the heavy end of the scale; light-duty electric lines at the other. The sizing itself is licensed engineering, done from the equipment schedule — never guessed from floor area.

From Restaurant Kitchen MEP Requirements — the invisible half of the build — the full reading, with the method behind this answer.