GEMARA

2a - Mishnah: A Sukah more than 20 Amos tall is Pasul;

R. Yehudah is Machshir.

Question: Why do Chachamim disqualify it?

Answer #1 (Rabah): "Lema'an Yed'u Doroseichem Ki va'Sukos Hoshavti" - Chachamim permit a Sukah up to 20 Amos, for then people will notice the Sechach.

If the walls reach the Sechach it is Kosher even if it is taller than 20 Amos, for people will notice the Sechach.

Answer #2 (R. Zeira): "V'Sukah Tihyeh l'Tzel" - if it is up to 20 Amos a person is in the shade of the Sechach. If it is above 20 Amos he is in the shade of the walls.

If the Sukah is wider than four Amos, it is Kosher even if it is taller than 20, for there will be shade from the Sechach.

Answer #3 (Rava): "Ba'Sukos Teshvu Shiv'as Yomim" - the Torah requires us to leave Diras Keva for seven days and live in Diras Arai. Up to 20 Amos a person builds a Sukah Arai. Above this he builds it Keva.

According to Rava any Sukah taller than 20 Amos is Pasul.

7b - Abaye: The following require (or allow) Sukah to be Diras Keva - Rebbi, R. Yoshiyah, R. Yehudah, R. Shimon, R. Gamliel, Beis Shamai, R. Eliezer and Others...

Chachamim disqualify a Sukah above 20 Amos; R. Yehudah is Machshir.

11a - Rabah bar Rav Huna: One may sleep under a canopy in a Sukah, even though it has a proper roof 10 Tefachim tall.

Suggestion: This is like R. Yehudah, who says that an Ohel Arai (the canopy) does not Mevatel an Ohel Kavu'a (the Sukah):

Mishnah - R. Yehudah: We used to sleep under the bed in the Sukah in front of Chachamim.

Question: Why didn't Rabah bar Rav Huna simply say that the Halachah follows R. Yehudah?

Answer: If so, one might have thought that R. Yehudah permits regarding a bed, for one sits or lies on top of it, but not regarding a canopy, which is used to cover;

Therefore Rabah taught regarding a canopy, to teach that an Ohel Arai does not Mevatel an Ohel Kavu'a even regarding a canopy.

Yoma 10b - Rava: R. Yehudah requires a Sukah to be Diras Keva, therefore he obligates a Mezuzah. Chachamim require Sukah to be Diras Arai, therefore they exempt from Mezuzah.

RISHONIM

Rashi (DH Ad): Rava explains that the Torah is concerned only that the Shi'ur of (the height of) a Sukah allows it to be Arai. If it is not too tall, it may be Keva.

Ritva (DH Amar): Tosfos says that this is based on R. Zeira's law, Kol she'Einah Ra'uy l'Bilah Bilah Me'akeves Bo. This is wrong. Whenever R. Zeira's law applies, the Gemara mentions it. Also, R. Zeira himself disagrees with Rava because of Abaye's question, that a small Sukah may be Keva.

Tosfos ha'Rosh (7b DH Kulhu): R. Yehudah allows a Sukah to be more Keva than Chachamim do. He also allows Diras Arai.

Rif (Sukah 3a) and Rosh (1:10): The following require Diras Keva - Rebbi, R. Yoshiyah, R. Yehudah, R. Shimon, R. Gamliel, Beis Shamai, R. Eliezer and Others. Normally, when we list Tana'im that hold similarly, the Halachah does not follow any of them. However, here the Halachah follows two of them, but not because we require Diras Keva. A Sukah with sloping walls is Pasul, and we decree to disqualify eating from a table outside the Sukah lest one be drawn after the table.

Rif (1a) and Rosh (1:1): A Sukah more than 20 Amos tall is Pasul. Some say that the Halachah follows Rabah, for he (almost always) followed Rav. It appears that the Halachah follows Rava, for we hold that Diras Arai is required; the Halachah does not follow R. Yehudah, who requires Diras Keva. All the more so it is Keva if the walls reach the Sechach! Also, the Sugya of a Sukah on top of a Sukah (10a) is like Rava.

Ran (DH Ika): Rabah followed Rav when Rav argued with Shmuel, but not in general. If a Talmid argued with his Rebbi in the life of the Rebbi, the Halachah follows the Rebbi (e.g. Rabah against Rava). Since the Rebbi did not retract, the Talmid's opinion is rejected. If the Talmid argued after the Rebbi died, perhaps the Rebbi would have agreed with him. Then, we follow the Talmid, for he is the latter opinion. Rabah bar Rav Huna and Abaye hold like Rava. According to Rabah and R. Zeira, there is no source that R. Yehudah requires Diras Keva!

Rebuttal (Tosfos 7b Kulhu): Granted, Abaye said that R. Yehudah requires Diras Keva, like Rava explains (anything above 20 must be Keva). But he challenged Rava! He holds that Chachamim agree, and they disqualify for the reason of Rabah or R. Zeira.

Rebuttal (of Tosfos - Rosh ibid.): Indeed, Abaye challenged Rava. However, he accepted Rava's answer! We have no proof whether or not Abaye adopted Rava's opinion.

Gra (OC 633:1 DH Bein): Surely, Abaye adopted Rava's opinion! Abaye said that seven Tana'im require Diras Keva, i.e. and the Halachah does not follow them. If also Chachamim in our Mishnah require Diras Keva, the Halachah is not like either Tana in our Mishnah!

Note: According to one answer of Rava (21b), both opinions in the Mishnah (20b) require Diras Keva! That answer must hold that this is the Halachah, and Abaye disagrees.

Ritva (DH Ad): 'Up to 20 Amos a person builds a Sukah Arai' connotes that even above this it is possible to build it Arai. However, normally this is not done. If one does so, his eccentricity does not help (it is judged like a tall Sukah of normal people, i.e. Keva).

Rambam (Hilchos Sukah 4:1): A Sukah cannot be more than 20 Amos tall. If it is, it is Pasul. It must be at least seven Tefachim wide. It can be as wide as desired.

Magid Mishnah: This is like Rava.

Ritva (DH Rava): Since it does not say 'Teshvu Shiv'as Yomim ba'Sukos', rather, "Shiv'as Yomim Teshvu ba'Sukos", this teaches that the Sukah must be made for only seven days, i.e. Arai.

ACHARONIM

Shulchan Aruch (OC 633:1): A Sukah more than 20 Amos tall is Pasul, whether it is wide or narrow, whether or not the walls reach the Sechach.

Taz (1): A Sukah above 20 Amos must be Kavu'a, and we hold that Sukah must be Diras Arai. If it is at most 20 it is Kosher even if it was built to be Kavu'a.

Gra (DH Sukah): Rabah bar Rav Huna holds like R. Yehudah, but the Halachah follows the Amora'im who hold like Chachamim, i.e. Abaye and Rav Yosef (19b), Rav (26a) and Shmuel (10b).

Gra (DH Bein): The Halachah follows Rava, for the Stam Gemara (11a, 21b and 10b) is like Rava.