号 (hào) — (classifier) used with numbers to indicate order, dates, or quantity of people; (noun) size, especially of clothing or shoes

号 has two main uses: a classifier for order/dates/people (e.g. 一号, 十月号, 一百多号人), and a noun for clothing/shoe size (大号 'large'). Compare 大小 or 尺寸 (general size) — 号 is specifically what you ask for when buying clothes or shoes.