Peah, Mishnah 6. This mishnah discusses the size of a field that is obligated in peah. Not every plot of land is large enough to carry the obligation of peah, and that is the subject of our mishnah.
The views of the Tanna'im regarding the size of the field:
Rabbi Eliezer: Land in which a quarter of a kav of seed can be sown - that is the minimum size for the obligation of peah. The dimensions of such a plot: ten and a fifth amos by ten and a fifth amos, a little more than one hundred and forty square amos, roughly thirty five meters.
Rabbi Yehoshua: The measure is not determined by the amount of seed sown in the field, but by the yield - a field that produces two se'ah of grain. This measure should not be confused with the "beis se'asayim" discussed elsewhere, which is based on the amount of seed sown; here the determination is made according to the yield.
Rabbi Tarfon: Six tefachim by six tefachim - that is what counts as a field with regard to peah. This measure appears elsewhere as well: in Maseches Kilayim we will learn that the standard garden bed is six by six, and that size is sufficient for sowing five different species in it without violating the prohibition of kilayim.
Rabbi Yehudah ben Beseira: The field must be large enough that what grows in it will be sufficient to reap a handful and then reap again - about two fistfuls of grain. The halachah follows Rabbi Yehudah ben Beseira.
It is worth noting: we do not rule halachah on the basis of what is written in a mishnah, and even when the mishnah states "the halachah is like his words," we do not follow that statement, since it too comes from the mishnah itself. The words of the Gemara or of the later authorities are needed in order to determine what the halachah is.
The view of Rabbi Akiva - "ein shiur":
Rabbi Akiva says "ein shiur" - there is no minimum size; even the smallest amount of land is obligated in peah. And not only that, but he lists a series of laws in which "land of any size at all" is relevant:
Peah - even the smallest piece of land is obligated in peah.
Bikkurim - if such land produces new fruit, bikkurim may be brought from it.
Prozbul - a prozbul may be written on the basis of land of any size at all.
Kinyan agav - land of any size at all can serve to acquire movable property along with it.
To understand the matter of prozbul:
When a prozbul is written, it is the lender who writes it. However, in order for the prozbul to be valid, the borrower must own at least land of any size at all. In this way it is considered as though the lender holds a pledge, a form of security against which he can collect, and this removes the Torah level prohibition of collecting after Shemittah. Therefore the borrower is required to own land of some size.
To understand kinyan agav:
Movable property is acquired through meshichah, by actually taking physical hold of it, whereas land is acquired through money (an exchange of money), through a document (the transfer of a deed), or through chazakah (performing an act on the land that demonstrates ownership). Kinyan agav is a form of attachment: although these modes of acquisition are not effective on their own for movable property, when a person takes hold of the movable property and at that same moment acquires land, the acquisition of the land can draw the movable property along with it. Land of any size can accomplish this, and that is the intent of the mishnah: this small piece of land, even of any size at all, acquires movable property along with it - through money, through a document, and through chazakah.
In summary: In this mishnah the Tanna'im disagree about the minimum size of a field that is obligated in peah - Rabbi Eliezer according to the amount of seed sown (a beis rova), Rabbi Yehoshua according to the yield (two se'ah of grain), Rabbi Tarfon according to the size of a garden bed, six by six tefachim, and Rabbi Yehudah ben Beseira according to the measure of reaping a handful and reaping again, and the halachah follows his view. Rabbi Akiva holds that there is no minimum size at all, and he listed the laws in which land of any size at all is effective: peah, bikkurim, the writing of a prozbul, and the acquisition of movable property along with land.