Siman 396

Every person has a Shabbat techum. And besides the techum, even one who went outside the techum has four amot, as it was taught (48b): "Abide every man in his place"—as under him, and this is the measurement of a person when he stretches out his hands and his feet. And behold, Moshe said this to those who went outside the techum to gather the manna. (Tur) And they measure for every person according to his own amot, and if he was a dwarf in his limbs, they give him four amot of an average person, and this has already been explained in siman 349. And these amot are measured for him generously and not precisely, and above in siman 363, section 34, we explained the reason for the matter; see there.

The Tur wrote that the Rif ruled that he has only four amot and he is in their center, meaning two amot to each side, and the Razah ruled that he has four amot in every direction, and they are eight by eight. And these words apply to walking, but regarding carrying—he has only four, thus far his words. But the Rambam in chapter 12, law 15, wrote that he has four amot and he stands at their side, and afterward he wrote that if there were two, and part of the four amot of this one enters into those of that one… if there were three and the middle one was swallowed between them, see there, and so wrote the Tur and the Shulchan Aruch above in siman 349; behold it is explicit that he does not hold that he is in their center, for if so, you would not find it possible for the middle one to be swallowed between them. Rather, he holds that he has four amot toward whichever side he desires. And according to this, one must wonder about the Tur and about our teacher, the Rema, who cited his words, for behold he does not hold so. (Magen Avraham, subsection 1) And it must be said that in truth he does not hold like the Rif but like the Razah, and the Razah does not hold that he is in their center, yet some say that even the opinion of the Rif is not so. (Taz)

Any place that is enclosed for the purpose of a dwelling is considered as four amot, and if he was there at the time the day was sanctified, that is, at the onset of the Shabbat—he is permitted to walk throughout this entire place, even if it is very large. Therefore, if the day was sanctified while he was in a city enclosed by a wall, even one as large as Antioch—he may walk through the whole of it, for it is only as four amot, and even if the place is currently desolate of dwellings. And if it is not enclosed for the purpose of a dwelling and it is a distinct place, such as a high mound or a karpef that was not enclosed for the purpose of a dwelling, or harvested standing grain where stalks enclose it, and he made them with palm branches and laurels so they would not move by the wind, if they are less than a beit satayim or exactly a beit satayim—they are considered as four amot. And more than a beit satayim—he has only four amot.