Gemara

6b (Mishnah): If a Nazir became Tamei on day 30, this nullifies the entire Nezirus.

R. Eliezer says, he must observe the restrictions of a Nazir for seven more days.

60b (Beraisa - R. Shimon): At the end of Tzara'as, a Metzora shaves in order to grow his hair again (so he can shave again after counting seven days).

Nidah 52b (Mishnah - R. Yishmael): If the ends of two hairs can be bent back to the roots, they disqualify a Parah Adumah (if they are not red); they are a Siman Tum'ah (or Taharah) of Tzara'as (depending on their color); they are considered hairs for everything (adulthood, the Isur to cut the sideburns, shaving of a Nazir...)

R. Eliezer says, the Shi'ur is to be able to grasp them with his fingernails;

R. Akiva says, the Shi'ur is to be able to cut them with a Zug (scissors).

(Rav Chisda): We are stringent like all of these opinions.

Rishonim

Rif (cited by Nimukei Yosef Yevamos 41a) and Rosh (Nidah 6:4): Rav Chisda is stringent because he is unsure whom the Halachah follows. Therefore, a 12 year old cannot do Mi'un (annul a Rabbinic marriage) once two hairs can be gripped with fingernails. She cannot do Chalitzah until they can be bent to the roots.

Tosfos Rid (Nazir 6b DH R. Eliezer): A Nazir Tamei shaves for Tum'ah, and again for Taharah. R. Eliezer requires seven days in between, so the hairs will be long enough for shaving to apply. This is why a Metzora waits seven days between the two times he shaves, so the hairs will be long enough to bend to the roots.

Rambam (Hilchos Ishus 2:16): The hairs regarding a boy or girl and everywhere are long enough to bend the ends to their roots. If they are shorter, but can be cut with a Zug, we are stringent in every case. Therefore, we are stringent to treat a boy or girl (past 13 or 12 years) with two hairs this size like an adult, and we are stringent to treat him or her like a minor until the ends can be bent to the roots.

Magid Mishneh: The Rambam holds that the smallest Shi'ur is long enough to be cut with a Zug. Rashi says that it is so one can grip them with fingernails.

Question: How can they argue about something that can be easily seen?

Answer #1 (Chidushei Anshei Shem Yevamos 41a): The proper text of Rashi (52b DH she'Yehu) calls what can be cut with a Zug the smallest Shi'ur.

Note: Rashi (52b DH Lehachmir) connotes that what can be gripped with fingernails is the smallest Shi'ur.

Answer #2 (Prishah EH 155:39): The Rambam explains that Zug (with which they can be cut) is a razor, therefore he says that this is the smallest Shi'ur.

Answer #3 (Chasam Sofer OC 154 DH veha'Mishneh): Rashi explains that R. Eliezer refers to hair that enters between the fingernail and skin and can be felt. The Rambam explains that he refers to gripping hair with two fingernails.

Question (Mishneh l'Melech): Why did the Rambam say that the Shi'ur everywhere is to bend the ends to their roots? We are stringent once they can be cut with a scissors! Why did he say that we are stringent until one can bend the ends to the roots? After this, the child is a Vadai adult!

Answer #1 (Ma'adanei Yom Tov Nidah 6:50): The Rambam explains that R. Akiva and R. Eliezer argue about R. Yishmael's Shi'ur of long enough to bend to the root, if this is when we hold the hair with fingernails, or with a scissors.

Rebuttal and Answer #2 (Mishneh l'Melech): If so, the Rambam should have said that it is a Safek until one can bend them with fingernails! Also, he says that hairs on a Parah that cannot be cut with a Zug are meaningless. Rather, there are three Shi'urim. Hairs are Vadai if they can be bent to the roots. There is a Safek from when they can be cut with a Zug until this length.

Rambam (Hilchos Tzara'as 2:1): Two white hairs are a Siman of Tum'ah for Tzara'as. They must be long enough to be cut with a Zug. If the root was black, even if they whiten later he is Tahor. If the root was white for any length, even if they are black past this, he is Tamei.

Rambam (Hilchos Parah Adumah 1:4): The hairs on a Parah Adumah must be red at the roots, long enough to be cut with a Zug. (It is not a problem if after this they are not red.) Less than this has no meaning. Therefore, if it had two white or black hairs too short to be cut with a Zug, it is Kosher.

Question #1 (Kesef Mishneh): The Halachah is stringent like all opinions. The Rambam holds that long enough to be cut with a scissors is the smallest Shi'ur, therefore it is Metamei in Tzara'as However, to be Metaher a Nesek, he requires hairs that can be bent to the roots, the longest Shi'ur. Here, why does the Rambam use the smallest Shi'ur to be Machshir?

Answers #1,2 (Kesef Mishneh, citing Ri Korkus): The Rambam holds that we are stringent like all opinions only for adulthood. For Parah, we hold like R. Akiva. The Rambam is stringent to say that black hairs of the smallest Shi'ur disqualify. If they are red at the roots for the smallest Shi'ur, this suffices so that we do not ignore the red, so the hairs do not disqualify.

Rebuttal (Lechem Mishneh Hilchos Ishus 2:16): He is stringent not only for adulthood. He says that hairs are Metaher a Nesek only if we can bend them to the roots! In Hilchos Ishus, the Rambam said that this is the Shi'ur everywhere! Perhaps the latter answer is correct.

Question #2 (Lechem Mishneh): The Rambam should have explained in Hilchos Tzara'as that the smallest Shi'ur makes only Tum'as Safek (so we do not burn Terumah, and he does not bring a Korban Asham)! Also, why is he concerned only for the color of the root, for any length? For Parah, he requires the red to be long enough to be cut with a scissors!

Answer (Lechem Mishneh): For Parah, since there is a hair, we require the Shi'ur of a black hair. For a hair white at the root, removing the root would be like removing the entire hair. Perhaps the Rambam relied on what he wrote here that we are stringent for all the Shi'urim.

R. Chaim ha'Levi (on Hilchos Ishus 2:16): The Halachah is to be stringent like all opinions. This is not due to Safek. Being an adult has leniencies and stringencies, therefore we are stringent in both directions as if it were a Safek. Regarding Tzara'as, it is usually a stringency to be considered a hair, therefore we follow the smallest Shi'ur even when it is a leniency.

Poskim

Shulchan Aruch (EH 155:18): Once two hairs sprout and can be cut with a scissors or grasped with fingernails, a 12-year old may not do Mi'un. She is not an adult to do Chalitzah until one can bend the end of the hairs to their roots.

Question (Yam Shel Shlomo Yevamos 12:26): Why did the SMaG and Tur bring the Perushim of Rashi and the Rambam? In any case we are stringent like all the opinions!

Answer #1 (Bach DH ul'Inyan): We do not follow a lone opinion against the majority. L'Chatchilah, we are stringent for all three opinions. If she did Mi'un or Chalitzah, she was an adult if her hairs satisfied two of the three opinions.

Answer #2 (Taz 9): They teach that we must check both with a scissors and with fingernails before allowing Mi'un.

Answer #3 (Hagahos Tur ha'Shalem): According to the Prishah (39), we are stringent for the Rambam's explanation that Zug is a razor.

Question: Perush Seder Chalitzah (of the Beis Yosef, 169:27) requires hairs as long as the width of a (middle) finger!

Answer #1 (Bach DH v'Chol): We are not proficient to know whether or not hairs can be bent. If they are as long as a finger (width), surely they can be bent. My Rabbis did not permit Chalitzah of a 13 year old even with two hairs a finger long, for the hairs must come from cavities, and we are not proficient about this. We require many hairs, or very long hairs typical of older adults. It seems that the Beis Yosef calls a finger 'very long'. One may rely on if needed to avoid Igun (prolonged inability to remarry).

Answer #2 Taz (10): If we know that he or she is 13 or 12, and that there are cavities, bendable hairs suffice. If not, they must be a finger long.