The word "大号" is formed by directly combining the character "大" (dà), meaning "big" or "large", with the character "号" (hào), meaning "number", "size", or "designation", creating a compound where "大" modifies "号" to convey the concept of "large size" or "large number", which is applied in contexts like clothing sizes or, colloquially, as a euphemism for a toilet.