Gemara

(R. Avahu): Whenever Chachamim gave a Shi'ur, we are always stringent about the exact amount, except for a blood stain the size of a bean. We are lenient about this.

(Mishnah): If the kidneys were removed (it is Kosher.)

55b: If the kidney shrank, if it shrank to the size of a bean in a small animal, or to the size of a grape in a large animal, it is Tereifah.

Rishonim

Rambam (Hilchos Shechitah 8:26): If the kidney is too small, and it decreased to the size of a bean in a small animal, or in a big animal to the size of a grape, it is Tereifah.

Rosh (Chulin 3:45): This is a Torah Isur, so we say 'Ad and including'; if exactly a bean or a grape remains, it is Tereifah. 'Decreased' connotes that it was big and decreased, and the decrease is evident in its place. If it was initially small, it is Kosher.

Ran (Chulin 17a): Bahag says that if the kidney decreased to the size of a bean or a grape, it is Kosher. Less than this is Tereifah. This is astounding. The Gemara said that all Shi'urim of Chachamim are to be stringent! For Torah laws, we say that 'Ad (until)' includes the limiting case if this is a stringency! Therefore, if it is exactly this size, it is Tereifah. The Ra'avad says that it is Tereifah only if it was normal sized, and decreased due to illness. If it was initially less than a bean, it is Kosher. If we see that the shell of the kidney is shriveled, we know that it is due to illness. If it is not shriveled, we know that it was initially small, and it is Kosher.

Poskim

Shulchan Aruch (YD 44:5): If a kidney of a small animal shrunk to the size of a bean or less, or a kidney of a big animal shrunk to the size of an average Anavah or less, it is Tereifah.

Beis Yosef (DH Kosav Mahari, citing Mahari Ben Chaviv): I heard that the kidney of a large animal is composed of separate layers. Each is called Anava. I remember seeing that Talmidei R. Yonah explained that Anava is Pigun (in Ladino?), which are slightly round, long and kidney shaped. It seems that they were bigger than a bean in the days of the Gemara, unlike nowadays. There is somewhat of a proof for this from what he wrote that a bird's spleen is round like an Anava.

Note: 'What he wrote' connotes that he cites Talmidei R. Yonah regarding a bird's spleen. The Rambam (Hilchos Shechitah 10:10) and Tur (YD 43, citing the Rashba) say so.

Yam Shel Shlomo (Chulin 3:88): Surely, Anava refers to a grape, like the simple meaning. Most Meforshim hold like this, therefore, they did not comment about this. This is not astounding if it is bigger than a bean. The Peros of the seven species of Eretz Yisrael were much bigger, based on nature. Tosfos says that an olive is half a chicken egg, even though now we do not find this. Rather, they changed.

Mishbetzos Zahav (YD 44:12): This is difficult. Perhaps we use their Shi'urim (from the days of the Gemara)! It is difficult to say that also the nature of animals changed (and if the size of a current grape remains, it is Kosher).

Shach (12): Ra'avan says that in Eretz Yisrael, the grapes were bigger. A case occurred in Magentza in which a big animal's kidney shrunk to the size of a grape, and they were Machshir.

Sifsei Da'as: We rely on our grapes, even though they are smaller.

Hagahos Chasam Sofer (on Shach): This is astounding. Why were they Machshir based on our grapes?

Ha'arah (on Tzlach Pesachim 116b DH v'Ho'il, 6): All Shi'urim are based on the current time and place. Ra'avan said that in Eretz Yisrael the grapes were bigger. In Magentza, how did they estimate a grape to be Machshir?

Shach (13): Toras Chatas (89) brings from Mahari Ben Chaviv that we do not discuss the size of a grape, rather, a layer of the kidney. This is astounding. He concluded that it is the size of a grape!

Pri Chodosh (9): We learned 'in a small animal the size of a bean, or in a big animal the size of an Anava Beinonis. Beinonis (average) was not taught regarding a bean. This connotes that even a small bean suffices. If so, it is not difficult to say that an average grape is bigger than a small bean. However, perhaps 'Beinonis' refers also to bean. We can say that the grapes in Eretz Yisrael were bigger (Kesuvos 111b). The Shach says that Mahari Ben Chaviv concluded that it is the size of a grape. This is wrong. Pigun is not a grape! I hold that the primary Perush is that it is a grape, and so explain the Maharshal, Bach, and Ra'avan.

Note: If Pigun is a grape, presumably Mahari Ben Chaviv suggested that it got smaller, for we have sources to say that grapes used to be bigger. If it is not a grape, why does he say that Pigun got smaller? Perhaps beans got bigger! Perhaps since we know that Peros of Eretz Yisrael decreased, it is unlikely that other Peros increased. Alternatively, it is unreasonable that a kidney smaller than the size of a bean nowadays suffices. Perhaps he knew of many animals that gave birth, and died soon after and were found to have kidneys slightly bigger than a bean, but one was never found in which it was less than a bean (for a Tereifah cannot gave birth).

Bach (DH Kosav ha'Ri): Mahari Ben Chaviv's text said 'Anava' with an Aleph at the end, therefore, he gave different explanations. Our texts say 'Anavah' with a Hei at the end, which is surely a grape. We must say that grapes used to be bigger, just like Mahari Ben Chaviv needed to say that Pigun used to be bigger.

She'alas Ya'avetz (1:69): We may derive that a Stam bean of the Gemara refers to the largest species of bean. We see that other beans are smaller than grapes. Even though this relies on Mahari Ben Chaviv, it seems that he had a tradition for what he said.

Aruch ha'Shulchan (23): If it is a Safek if the Shi'ur remained, it is Kosher due to a Sefek-Sefeka (two doubts). Perhaps a Shi'ur remained, and even if it did not, perhaps this was from birth (and not due to illness). However, I do not know if we know the Shi'ur of a grape, since it is unlike grapes nowadays. If so, who can judge for a big animal, unless it is clear to the eye that it is the proper Shi'ur. This requires investigation.

Kaf ha'Chayim (52): Presumably, the remnant must be kidney shaped. If it is elongated or flattened, it is a Safek.

Shulchan Aruch (ibid.): This is only if it decreased due to illness. If it was initially small, it is Kosher. If we see that the shell is shriveled, we know that it is due to illness. If it is not shriveled, but it is like the size of a kidney, this shows that it was initially small, and it is Kosher.

Pischei Teshuvah (7): Shevus Yakov (2:62) was asked about an animal with a very big kidney and a very small kidney. They did not know if it was initially like this. He answered that just like one was big not due to illness, rather, it was initially big, we say that the other was initially small. Gedolim agreed with him.

Shulchan Aruch (6): Some say that that if the kidneys were removed it is Kosher, i.e. they were removed by hand or it was born like this. If they dissolved due to illness, once they became less than the Shi'ur, the animal was Tereifah, and it does not become Kosher again when the kidney totally dissolves. Based on this, we must check the sac to see if it is full of Chelev. This shows that it was initially like this. If there is a cavity, this shows that it had kidneys but they dissolved, so it is Tereifah.

Source (Beis Yosef DH Kosav ha'Kol, citing the Kol Bo): According to the Ra'avad, we needed to explain that if the kidneys were removed it is Kosher, i.e. it was born like this. If they disappeared due to illness, once they became less than the Shi'ur, the animal was Tereifah, and it does not become Kosher again when the kidney totally dissolves. If so, we must check the sac to see if it is full of Chelev and has only strands. This shows that it was initially like this. If there is a cavity, this shows that it had kidneys but they dissolved, so it is Tereifah.

Rebuttal (Yam Shel Shlomo Chulin 3:88): The Kol Bo would be stringent also if there is only one kidney. This is wrong. The Mishnah is Machshir if a kidney was removed! This refers to through man or through sickness. The Rambam (Halachah 24) explicitly says so. We find that a punctured lung is Tereifah, but when it becomes covered by flesh, it is Kosher! Only regarding man's actions we say that it does not become Kosher again, e.g. if the leg was cut at the juncture of the sinews (it is Tereifah, even if one will later cut it above, even though normally cutting above does not make Tereifah).

Defense (Taz 13): Sometimes we find that a Tereifah becomes Kosher again. What forces us to say that Chachamim permitted here?! We can explain that 'Nitlu' (they were removed) means from birth! What is the source to say that it is even through illness, since it became Tereifah in the middle! This is unlike a punctured lung that became covered with flesh. Surely, once it is healthy, it is not Tereifah. If we would know every punctured lung will became covered, we would not call it Tereifah. Since we do not know, we call it Tereifah. Here, once it shrivels due to illness, why should we call it Tereifah (according to Kol Bo)? We know that it will totally dissolve, and the animal will return to health! Do not say that perhaps the kidney will heal and not totally dissolve. If so, all the more so it should not be Tereifah, for it will return like initially! Rather, it is Tereifah because it will totally dissolve.

Defense (Shach 30): A punctured lung is different. It becomes Kosher only when the wall covers it, and it is as if it was sealed from the beginning.