The first Mishnah in Succah discusses the height of Schach but not the measures. Subsequently, the Mishnah does not deal with it at all (Mishna 2.7 discusses the placement of the table independently of the size of the Succah)
Why does the Mishnah omit such a fundamental Halacha?
N.b. the Halacha is finally derived by the Gemmorah, but the question is why not by the Mishnah?