Gemara

18a (Mishnah): One may distance Pasei Bira'os as far [from the well] as desired;

R. Yehudah permits up to Beis Sa'atayim.

Chachamim: Beis Sa'atayim is the limit for a garden or Karfef (an enclosed area without a roof). There is no limit for a Dir (a field where one grazes animals to fertilize it), Rechavah (a Chatzer in back of houses) or Chatzer.

23a (Mishnah): R. Yehudah ben Bava taught that one may carry in a garden 70 Amos and a fraction by 70 and a fraction, as long as there is a Shomirah (a watchman's shack) or Beis Dirah (house for residence) inside, or it is close to the city.

R. Yehudah says, even if there is only a pit or cave inside, one may carry inside;

R. Akiva says, even if none of these is inside one may carry inside, but only if it is 70 Amos and a fraction by 70 and a fraction.

23b - Question: R. Akiva holds just like the first Tana!

Answer: They argue about a small amount.

(Rav Bivi): The Halachah follows R. Akiva.

24a (Rav Nachman): If one made an opening [from his house] to a Rechavah, he may carry in all of it.

This is only if he opened and later surrounded it, but not if he surrounded and later opened.

Rishonim

Rosh (2:2): Rashi explained that Shomirah and Beis Dirah are considered Hukaf l'Dirah (surrounded for the sake of residence). This implies that it was opened, and later Hukaf. However, according to what I (some fix the text to say "Tosfos") explained, a Shomirah or Beis Dirah is an inferior dwelling, and it helps even if it was Hukaf and later opened, up to Sa'atayim. For more than Sa'atayim, we require opened and later Hukaf.

Tosfos (23a DH u'Bilvad): Even if it is less than 70 Amos [and...], it needs a Shomirah or house. If it is more than Sa'atayim, a Shomirah or house does not permit. It is permitted only if the Karfef was opened and later Hukaf for the sake of the house. Do not say that the Shomirah is needed only for when it is 70 Amos, but nothing is needed if it is smaller, and if it is bigger, a Shomirah does not permit. It is unreasonable that there are three different laws for three areas that differ by Mashehu (a tiny amount)! Also, do not say that a Shomirah permits more than 70 Amos, and [R. Akiva] teaches only that nothing is needed for less than 70 Amos. If so, how does R. Akiva argue with R. Yehudah ben Bava, according to R. Chananel, who says that R. Yehudah ben Bava discusses a small amount [above 70 Amos and four Tefachim], and R. Akiva does not? Both of them require a Shomirah if it is more than 70 Amos and a fraction, and do not require [anything] if it is less than this (70 and a fraction)! Even according to Rashi, who says that the Gemara asked that R. Akiva is like the first Tana [on 18a, who permits more than Beis Sa'atayim in an area Hukaf l'Dirah], it seems that R. Yehudah ben Bava discusses a small amount [above 70 and four Tefachim], like the first Tana. This is why R. Akiva had to repeat 'but only if it is 70 Amos and a fraction.' Also, if R. Yehudah ben Bava discusses more than Sa'atayim, why does R. Yehudah permit 'even if there is only a pit or cave inside'? This implies that he equates more than Sa'atayim with Sa'atayim. He himself said above that Pasei Bira'os permit only up to Sa'atayim! Also, [if R. Yehudah ben Bava permits more than Sa'atayim,] according to R. Chananel, who says that we asked that R. Akiva is just like R. Yehudah ben Bava, i.e. why did he need to say 'but only if'? Since he addresses R. Yehudah ben Bava, R. Akiva needed to limit the Heter to Sa'atayim! Therefore, what I explained is correct. Rashi says that R. Yehudah ben Bava does not permit more than Sa'atayim even if it was Hukaf l'Dirah. This is wrong. If it was opened and later Hukaf, even more than Sa'atayim is permitted. Rabanan permit a Dir, Sahar or Muktzeh even the size of 60 Sa'im. Many Chatzeros and Mavo'os in Yerushalayim were bigger than Sa'atayim. R. Yehudah ben Bava will not forbid them to carry! He says only that a Shomirah does not permit more than Sa'atayim, but opened and later Hukaf does. Perhaps Rashi can distinguish, and say that a Dir, Sahar, Muktzeh or Chatzer have great usage, therefore even 300 Sa'im are permitted. However, a Karfef is permitted only up to Sa'atayim, even if it was opened and later Hukaf.

Hagahos Ashri (2:2, citing Or Zaru'a): If a garden or Karfef was surrounded by a wall 10 Tefachim tall, and there is a Shomirah or Beis Dirah, or it is near the city, since it is near his house, he intends to use it constantly, so it is as if it was Hukaf l'Dirah. It is permitted even if it is very big.

Poskim

Shulchan Aruch (OC 358:1): Any enclosure not for the sake of Dirah, e.g. a garden, orchard or shack merely for Shemirah, Chachamim forbade carrying more than four Amos in it if it is more than Sa'atayim. If it is Sa'atayim, one may carry in all of it. If it was Hukaf l'Dirah, even if it is several Mil, one may carry in all of it.

Rema: Some say that any Karfef close to the house is called Hukaf l'Dirah, for he intends for it.

Beis Yosef (DH b'Hagahos): Hagahos Ashri learns from R. Yehudah ben Bava in the Mishnah. Even though the Halachah follows R. Meir, who permits carrying in it even if it does not have any of these, we learn from R. Yehudah ben Bava that these are considered Dirah. Therefore, when it is more than Sa'atayim, and we require Hukaf l'Dirah, these are considered Dirah. Seventy Amos and a fraction according to R. Yehudah ben Bava are like more than 70 Amos and a fraction according to us. Tosfos disagrees. He says that R. Yehudah ben Bava holds that a Shomirah is not enough for more than Sa'atayim. Rather, the Karfef must be opened and later Hukaf for the needs of the house.

Magen Avraham (4): R. Yehonason says that it must be close to his house, i.e. within 2000 Amos, so he can go there on Shabbos. A guard's shack helps if he dwells there day and night, and a Beis Dirah helps even if he does not live there regularly (Hagahos Ashri, Hagahos Maimoniyos). He must discuss a house there from the beginning, for in 2:6 he says that a house built next to an orchard does not help. I say that also Tosfos wrote that it does not help only according to R. Yehudah ben Bava. We hold that less than 70 Amos does not require Shomirah, so it helps for more than 70. The Rema omitted Shomirah and Beis Dirah, for there is no argument about them.

Gra (DH v'Yesh Omrim Od): The same applies to a Shomirah or Beis Dirah (Hagahos Ashri). He learns from R. Yehudah ben Bava to how we hold.

Mishnah Berurah (16): This opinion holds that anything close to the city is called close to his house.

Mishnah Berurah (17): This opinion requires that the house preceded the wall. It permits even if it was not opened [to the Karfef]. Since it is very close to the city, he plans to use it constantly, so presumably it was Hukaf l'Dirah.

Rema (ibid.): Some argue about this.

Gra (DH v'Yesh Cholkim): Tosfos brought three proofs that [Shomirah] does not help according to R. Yehudah ben Beseirah when it is more than Sa'atayim, and the same applies to us. The last proof is also according to R. Akiva. The Gemara asked that R. Akiva is like the first Tana. [If R. Yehudah ben Bava permits even more than Sa'atayim,] R. Akiva needed to say 'but only if' to teach that he permits only up to Sa'atayim! (We would not suggest that he holds like the first Tana!) Rather, the first Tana never permits more than Sa'atayim, both according to Rashi that the 'first Tana' is on 18a, and all the more so according to Tosfos, that it is R. Yehudah ben Bava. Also R. Akiva always forbids more than Sa'atayim. The Beis Yosef and Bach agreed, unlike the Magen Avraham. The Rema omitted a Shomirah and Beis Dirah. He holds that a Shomirah does not permit, unlike Hagahos Ashri. In Sa'if 2, he mentions only a Dirah. See the Rosh, who says 'according to what I explained...' (Damesek Eliezer - he sides with Tosfos.) [Hagahos Ashri holds that] a Beis Dirah [permits even if it] was Hukaf and later opened. Surely we do not hold like him also regarding this. See the Magen Avraham.

Mishnah Berurah (18): This opinion holds that even if it is adjacent to his house, if it was not opened and later Hukaf, it does not help. The Acharonim hold like this.

Kaf ha'Chayim (25): The Machatzis ha'Shekel says that the Mechaber (501:3) connotes like Tosfos.