Al-sabʿ al-ṭuwal refers to the seven outstandingly long surahs. Their identity rests on length, not on the assumption that each has one exclusive subject. Legal guidance, belief, narratives, and ethical exhortation may all form parts of a coherent extended address.
Al-miʾūn is related to the Arabic miʾah, “one hundred,” and refers to surahs of roughly a hundred verses or somewhat more. This is a conventional approximation, not a requirement that each surah contain exactly the same number of verses.
Both labels concern size. They do not determine a surah’s full rhetorical structure; its various topics must still be studied in relation to one another.