Gemara

25a - Mishnah: One may eat and drink Arai outside the Sukah.

26a - Question: What is considered Achilas Arai?

Answer #1 (Rav Yosef): It is (bread the size of) three or four eggs.

Objection (Abaye): This is often enough for a person. It is a fixed meal!

Answer #2 (Abaye): It is the amount a Talmid tastes before going to learn (he is concerned lest the learning delay his normal morning meal).

26b - Mishnah: A case occurred, a cooked food was brought to R. Yochanan ben Zakai and two dates and a flask of water to R. Gamliel. They commanded to take the food up to the Sukah.

Food less than a Beitzah was sent to R. Tzadok. He held it in a cloth, ate it outside the Sukah and did not bless afterwards.

Question: The (first) case contradicts the law (taught right before it in the Mishnah, that one may eat and drink Arai outside the Sukah)!

Answer: The Mishnah is abbreviated, it means as follows. If one wants to be stringent, he may. This is not considered haughtiness. A case occurred...

Inference: R. Tzadok ate outside the Sukah because the food was less than a Beitzah. Had it been a Beitzah, he would have eaten it in the Sukah.

Suggestion: This refutes Rav Yosef and Abaye, who permit eating this amount or more outside the Sukah!

Rejection: Perhaps a Beitzah would obligate Netilas Yadayim and blessing afterwards (but it would still be permitted outside the Sukah).

27a: (R. Eliezer says that if one did not eat in the Sukah on the first night he should be Mashlim on the night of Shemini Atzeres.) He is Mashlim with kinds of Targima.

Support (Beraisa): If one was Mashlim with kinds of Targima he was Yotzei.

Yoma 79a - Beraisa: R. Yochanan ben Zakai and R. Gamliel were not obligated to eat the Tavshil and dates in the Sukah. They were stringent on themselves.

Inference: R. Tzadok ate less than a Beitzah outside the Sukah. A Beitzah must be eaten it in the Sukah. This implies that two dates are less than a Beitzah!

Rejection (Rava): Two dates are a Beitzah (or more). The Petur from Sukah is because fruits need not be eaten in the Sukah (not because of the Shi'ur).

Question (Beraisa - Rebbi): When we learned by R. Eliezer ben Shamu'a, they brought to us figs and grapes. We ate them Arai outside the Sukah.

Inference: Achilas Keva of fruits would require a Sukah!

Answer #1: It means, we ate them outside the Sukah like Achilas Arai (of bread).

Answer #2: They ate them Keva along with Achilas Arai of bread outside the Sukah.

Support (Beraisa): If one was Mashlim with kinds of Targima he was Yotzei.

If fruits require a Sukah, the Beraisa should teach a bigger Chidush, that one can be Mashlim with fruits!

Rejection #1:Targima are fruits.

Rejection #2: The Beraisa teaches about a place where there are no fruits.

Rishonim

Rif: The amount a Talmid tastes before going to learn is Arai. One may eat this much outside the Sukah.

Ran (DH Ma'aseh): The Rif omits the teaching that fruits never require a Sukah. The Ramban explains that once we answer that R. Tzadok could have eaten more than a Beitzah outside the Sukah, we need not say that fruits never require a Sukah. The Ramban also says that a Talmid tastes one or two Beitzim before going to learn. The Gemara in Yoma connotes that it is one Beitzah, like Rashi says.

Rambam (Hilchos Sukah 6:6): All seven days we eat and drink in the Sukah. One may not eat a meal outside the Sukah unless it is Arai, i.e. k'Beitzah or less or a little more. One may drink water and eat fruits outside the Sukah. It is praiseworthy to be stringent and not drink even water outside the Sukah.

R. Mano'ach: Even a k'Zayis of bread must be eaten in the Sukah. One blesses Leshev ba'Sukah for anything which requires the Sukah. The Isur of eating outside the Sukah is for a Beitzah. Fruits are exempt, but Targima, i.e. accompaniments for bread such as meat, cheese, eggs and fish must be eaten in the Sukah. It is good to be stringent to drink wine in the Sukah because a meal can be called Mishteh because of wine. However, really wine is like fruits. 'One who washes his hands for fruits is haughty' (Chagigah 18b) applies even to wine.

Note: The Gemara does not say that it applies to wine, but Tosfos (Berachos 43a DH Kol) says so.

Rosh (2:13): Arai is the amount a Talmid tastes before going to learn. This is a mouthful, i.e. the amount that one can swallow at once, which is a k'Beitzah. This applies to bread. One may eat (any amount of) fruits outside the Sukah. We learn from Yoma 79b that Keva of fruits is like Arai of bread. Rabeinu Meir was careful not to eat fruits outside the Sukah. He was stringent like the version that fruits must be eaten in the Sukah. However, this is only if one was Kavu'a to eat fruits. Drinking wine is Arai.

Korban Nesan'el (80): R. Yirmeyah and Rav Zevid (Yoma 79b) hold that fruits require a Sukah.

Rebuttal (Taz OC 639:8): No version in the Gemara requires a Sukah for fruits! Rava says that fruits do not require a Sukah. He is challenged, and two answers are given. Neither holds that fruits require a Sukah!

R. Peretz (brought in Rosh, ibid.): Fruits are Arai, but meat, cheese and other foods other than fruits must be eaten in the Sukah like we say here, that if one was Mashlim with kinds of Targima he was Yotzei. Targima is things like meat. Since it is compensation, this shows that it must be eaten in the Sukah. This is unlike Rashi, who says that Targima is fruits, which need not be eaten in the Sukah.

Rebuttal (Rosh, ibid.): A Tosefta in Berachos says that one blesses Borei Minei Mezonos on Targima. This shows that it is made from the five grains. Perhaps it is more important than meat and cheese, so we cannot learn from Targima to require a Sukah for meat or cheese. Also, it is not common to fix a meal on meat and cheese. It is no more fixed than Achilas Arai of bread.

Poskim

Shulchan Aruch (OC 639:2): We eat and drink in the Sukah. One may eat Achilas Arai outside the Sukah, i.e. a k'Beitzah of bread.

Mishnah Berurah (12) and Bi'ur Halachah (DH Aval): It is normal to eat Arai outside one's house. One who cooks in the house may taste and swallow food many times. Even a Chacham may eat Arai outside the Sukah, and he is not called one who is not meticulous in Mitzvos.

Shulchan Aruch (ibid.): One may drink water or wine and eat fruits (Rema - even Keva) outside the Sukah.

Magen Avraham (6): All agree that Achilas Arai of fruits is exempt. The argument is only about Keva. The Rosh holds that Keva of fruits is more than a Beitzah, for otherwise it would be like bread. The Gemara (Yoma 79b) refutes this. L'Ma'aseh, fruits are exempt in any case, but we learn that according to the opinion that Targima requires a Sukah, a Beitzah is Keva.

Beis Yosef (DH u'Mah she'Chosav Rabeinu): The Tur says that wine is like fruits, i.e. it requires a Sukah only according to the opinion that fruits require a Sukah.

Levush (brought in Taz 7 and Magen Avraham 5): The Darchei Moshe (4) says that nowadays there is no Kevi'us on wine, so it is exempt from Sukah. However, we hold that there is Kevi'us (OC 174:4 and 213:1). One who drinks to quench his thirst is exempt, but one who us Kove'a to drink much wine should be stringent. One may bless, for letter of the law one should bless whenever he enters the Sukah, just like it was established to bless on eating.

Rebuttal #1 (Taz 7): He should not bless. Even if wine requires a Sukah, the Berachah he will make on eating exempts it. The Berachah on eating exempts even sleeping, which always requires a Sukah!

Bach (DH u'Mah she'Chosav u'Shtiyas): When people drink wine together, this is Kevi'us. To avoid the argument (about whether or not one should bless), he should not be Kove'a to drink wine outside a meal. The same applies to meat and cheese.

R. Akiva Eiger: R. Avigdor says that on Shabbos and Yom Tov everything is Keva. Tosfos says that the Chiyuv to eat makes it Keva. If so, this does not apply after eating the Yom Tov Seudah.

Kaf ha'Chayim (34): On Shabbos and Yom Tov it is good to eat even Achilas Arai in the Sukah, but not to bless.

Shulchan Aruch (ibid.): It is praiseworthy to be stringent and not drink even water outside the Sukah. If one fixed a meal on a Tavshil made from the five grains this is Keva and it requires a Sukah.

Mishnah Berurah (15): Keva is eating with others, or eating enough to fix a meal on it. This is like the Shulchan Aruch and Acharonim, who oppose the Magen Avraham (6, brought above).

Sha'arei Teshuvah (3): On Shabbos and Yom Tov, if one is Yotzei Kidush b'Makom Seudah by eating more than a Beitzah of cake this is Keva. At other times, if one eats more than a Beitzah of cake, even if is not Kevi'us to bless ha'Motzi, some say that it requires a Sukah. To avoid a Safek Berachah l'Vatalah, one should not leave the Sukah immediately after eating. The Berachah 'Leshev' also includes staying in the Sukah afterwards.

Kaf ha'Chayim (33): The Chida says that Kidush is not Kove'a unless one eats bread or is Kove'a on cake. Where there is a custom (to bless for Kidush in any case), we do not say Safek Berachos Lehakel.

Kaf ha'Chayim (40): The Shi'ur for Kevi'us is based on most people, i.e. three k'Beitzim. If one eats less he should hear Leshev ba'Sukah from others or bless in his heart.