- The height:
Minimum height:[1] The Sukkah must have minimum height of 10 Tefachim [82 cm.[2]]. If its height is less than ten Tefach high, it is invalid. The height is measured from the floor of the Sukkah until the Sechach. Even if decorations hang within the ten Tefach space of the Sukkah, it remains valid.[3]
Maximum height:[4] The maximum permitted height that a Sukkah may have is 20 Amos [940 cm.[5] or 30.8 feet]. This applies irrelevant of the size of the Sukkah, whether it is large or small. This height is measured from the floor of the Sukkah until the Sechach.[6] This applies whether or not the walls of the Sukkah reach the Sechach. If there is more than 20 Amos between the floor and the Sechach, then it is invalid. If there is exactly twenty Amos, then it is valid. If the Sechach is above twenty Amos from the floor, but under the Sechach there is only twenty Amos of space between it and the floor [i.e. the Sechach is in the 21st Ama], then it is valid.
[1] Michaber 633:10; Sukkah 4b and 5b
[2] See Shiureiy Torah p. 249 that an Ama Sochekes is 49 cm., and accordingly a Tefach Sochakos is 8.17 cm [i.e. 1/48 of a cm times 8.] and by the Shiur Sukkah of 7×7 one should measure with an Ama Sochakos, and the same would apply here Lechumra. Thus, its exact Shiur is 81.7 cm; See also M”B 633:2 and Shaar Hatziyon 633:2 that by Sukkah we follow the stringent approach in the dimensions, either Dochakos or Sochakos, and by the 7×7 dimension we follow Sochakos, and the same would apply here by the ten Tefach dimension; See also Midos Vishureiy Torah pp. 48-57 regarding Ama Sochakos and Dochakos; Piskeiy Teshuvos 633:1; Minchas Chinuch Mitzvah 325 who rules that by Tefachim we don’t apply the extra Dochakos or Sochakos, and this is only done by Amos
Other opinions-Chazon Ish: According to the Chazon Ish, the measurement is 98.2 cm following the 9.82 measurement of Tefach Sochakos. [See Piskeiy Teshuvos ibid]
[3] See Michaber 633:9-10 and Sukkah ibid and 10b regarding if a) there are Sechach pieces and decorations that go into the ten Tefachim and b) if one dug out the floor of the Sukkah so it reach ten Tefachim.
[4] Michaber 633:1; Tana Kama in Mishneh Sukkah 2a
Other opinions in Mishneh: Rebbe Yehuda is of the opinion that a Sukkah which is higher than 20 hours is valid. [Mishneh Sukkah ibid]
Other details: Sechach is higher than 20 Amos, but it has pieces of it which reach within twenty: See 633:2; Its inside contains more than 20 Ama and hung decorations or covered floor with pillows or earth, or platform/Itztaba, which now makes it contain less: 633:3-7
[5] See Shiureiy Torah p. 249 that an Ama Dochakos is 47 cm. [as opposed to 48 for a regular Tefach], and by the Shiur Sukkah of 7×7 one should measure with the stringent approach of an Ama Sochakos, and the same would apply here Lechumra to measure the twenty Amos as Dochakos. See also M”B 633:2 and Shaar Hatziyon 633:2 that by Sukkah we follow the stringent approach in the dimensions, either Dochakos or Sochakos, and by the 7×7 dimension we follow Sochakos, and the same would apply here by the 20 Amos dimension; See also Midos Vishureiy Torah pp. 48-57 regarding Ama Sochakos and Dochakos; Piskeiy Teshuvos 633:1 writes that its 938 cm according to Grach Nah, taking into account the Dochakos according to Chazon Ish, as he explains there in footnote 2
Other opinions-Chazon Ish: According to the Chazon Ish, the measurement is 1,130 cm following the 9.42 measurement of Tefach Dochakos. [See Piskeiy Teshuvos ibid]
[6] The reason: The reason for this is because the Sechach is considered to be unable to provide shade when it is more than twenty Amos from the floor.