1) "KOL SHI'UREI CHACHAMIM L'HACHMIR"
OPINIONS: Rebbi Avahu says in the name of Rebbi Yochanan, "All of the amounts established by the Chachamim are in order to be stringent, except the Gris for Kesamim, which is to be lenient." This means that whenever the Chachamim established an upper (or lower) limit for an amount regarding a certain Halachah, that limit is included in the stringency of the Halachah ("Ad v'Ad Bichlal"), except for the Shi'ur of a blood stain the size of a Gris bean. In the case of a stain, the stain is considered Dam Nidah only when there is more than the size of a Gris. The limit of a Gris is not included in the stringency of the Halachah.
Does this mean that all Shi'urim are included in the Chumra element of the Halachah, or does Rebbi Avahu in the name of Rebbi Yochanan refer only to a specific category of Shi'urim?
(a) TOSFOS (DH Kol) clearly learns that when Rebbi Avahu says "Kol Shi'urei Chachamim," he refers to "all amounts" established by the Chachamim. Tosfos questions Rebbi Avahu's statement from the Gemara in Berachos (27a). Rebbi Yehudah maintains that one may recite Musaf until and including the seventh hour of the day. Accordingly, when Rebbi Yehudah says that one may recite Shacharis until the fourth hour, he must mean that one may recite Shacharis until and including the fourth hour. How, though, can Rebbi Yehudah say that the latest limit for praying is lenient and includes that hour, if, as Rebbi Avahu says, all of the Shi'urim of the Chachamim are l'Chumra? Tosfos answers that Rebbi Yehudah's ruling is also a stringency, because he is ruling that a person is obligated to pray through the fourth and seventh hours if he has not yet prayed. It is clear that Tosfos learns that Rebbi Avahu's rule refers to all types of Shi'urim, whether they are mid'Rabanan or mid'Oraisa.
The TIFERES YAKOV is perplexed by the answer of Tosfos. A "Chumra" means that a certain object has a status that limits its use. It is not a Chumra to say that a person is obligated to pray, since most people want to pray and do not view the obligation to pray as a limitation. Any normal person who wants to do the Mitzvos looks at the ability to continue praying passed a certain hour as a leniency, and not as a stringency. The opinion of the Chachamim in Berachos (26a), who say that one may recite Musaf all day, certainly is not considered an exceptionally stringent opinion. (See the Tiferes Yakov who has a different approach to the Gemara in Berachos.)
(b) The RASHBA argues that Rebbi Avahu means something else entirely. Rebbi Avahu refers only to amounts that are relevant to a Halachah mid'Oraisa. For Halachos mid'Rabanan, the amounts were given in order to be lenient.
According to this explanation, why does Rebbi Avahu himself say that the amount of a Gris with regard to Kesamim is an exception and is lenient? The Halachah of Kesamim (that a woman is considered a Nidah when she finds a stain and is unsure about where it came from) is mid'Rabanan. If Rebbi Avahu maintains that all Shi'urim regarding Halachos mid'Rabanan were given to be lenient, then he should have said simply that all amounts of the Chachamim are stringent except for those that apply to laws that are mid'Rabanan. Why does he specifically mention the exception of Kesamim?
The Rashba explains that this actually is Rebbi Avahu's intention with his statement. He mentions a Gris of Kesamim merely as an example of all amounts that apply to Halachos mid'Rabanan.
The ARUCH LA'NER in Nidah (58b) says that the Rashba understands that this is the opinion of RASHI (DH Chutz). Rashi explicitly states that the reason for Rebbi Avahu's Halachah is that the law of Kesamim itself is mid'Rabanan. The ROSH YOSEF here writes that he found that this is also the understanding of the RAN. Accordingly, all of these opinions are not bothered with Tosfos' question from Berachos, since the Gemara in Berachos refers only to amounts that involve Halachos mid'Rabanan. (Y. MONTROSE)
2) THE CRUCIAL AMOUNT OF SKIN
OPINIONS: In the Mishnah (54a), Rebbi Meir and the Chachamim disagree about the status of an animal that has had its skin removed ("Geludah"). Rebbi Meir says that the animal is Kosher, while the Chachamim say that the animal is a Tereifah. The Gemara here teaches that Rebbi Meir eventually retracted his opinion and agreed with the Chachamim.
The Gemara discusses how much skin of the animal must be removed in order for it to be considered a Tereifah due to Geludah. Rebbi Tarfon is quoted as saying that if an amount of skin the size of a Sela (coin) remains on the animal, then the animal does not have the status of a Geludah. The Gemara asks where on the animal must this Sela of skin remain in order for the animal not to be a Tereifah. The Gemara records a number of opinions.
Shmuel says that a width of Sela of skin must cover the entire spine. Rabah bar bar Chanah says that a Sela must cover the vertebrae and the joints. Rebbi Elazar ben Antignos says that it must cover the navel. Rav says that a Sela of skin anywhere on the body makes the animal Kosher, except over the Beis ha'Perasos (the feet, below the knee), and Rebbi Yochanan says that even skin over the feet makes it Kosher.
Which opinion does the Halachah follow?
(a) The Rif quotes the BEHAG who rules like Rav, that an amount of skin the size of a Sela on any part of the animal prevents it from having the status of a Geludah, except for the skin on the legs, below the knee. One of the proofs of the Behag is that Rebbi Yochanan himself retracted his opinion and agreed with the opinion of Rav.
(b) The RIF argues with the Behag and maintains that the Halachah should follow the view of Shmuel, that as long as there is a strip of skin along the entire spine of the animal which is not less than a width of a Sela at any point, the animal is not a Geludah. He says that there is no proof in the Gemara for which opinion to follow, and there is no indication in our text that Rebbi Yochanan retracted his opinion and agreed with Rav. Therefore, the Halachah should follow the most stringent opinion, that of Shmuel.
It is clear from the words of the Rif that he understands that all of the other opinions in the Gemara are not stating a specific part of the body where skin must remain in order for the animal to survive, but rather they are giving the most lenient place where such an amount of skin could save the animal. Thus, the opinion that says, for example, that the skin must cover the navel, maintains that a Sela of skin at the navel will prevent the animal from being a Tereifah, and certainly skin over the spine will prevent it from being a Tereifah (as Shmuel says).
(c) The ROSH (3:47) rules like the Behag that an amount of skin the size of a Sela on any part of the animal prevents it from having the status of a Geludah. However, while the Behag says that skin on the lower legs does not help, the Rosh rules that skin even in this area counts towards the amount of skin the size of a Sela.
(d) The RAMBAM (Hilchos Shechitah 9:7) learns differently. He rules that in order to avoid the status of a Geludah, an animal must have skin in all of the places mentioned by all of the opinions in the Gemara. The KESEF MISHNEH explains that the Rambam, like the Rif, is in doubt about whom the Halachah follows. However, in contrast to the Rif, the Rambam learns that each opinion in the Gemara is saying that the animal will live only when there is skin in this particular area. Therefore, the Rambam rules that the animal must have skin in all of these areas in order to avoid being a Geludah. This is also the opinion of the RASHBA, RAN, SHULCHAN ARUCH (YD 59:1), ARUCH HA'SHULCHAN, and most of the Poskim. (Y. MONTROSE)