Gemara

52b (Beraisa - R. Chanina ben Antignos): Kesamim do not make a woman a Zavah (if three were found in one garment).

(Rava): R. Chanina refuted the Chachamim (who disagree) as follows:

You agree that if she found a Kesem less than three Grisim (beans) in one place, she is not a Zavah, for we assume that it is from (at most) two sightings;

Likewise, if she found more than three Grisim in one place, we can attribute two and a half Grisim to two sightings, and the additional half-Gris to a louse, which was attracted by all the blood!

Chachamim hold that since there is enough to attribute more than a Gris to a sighting on each day, we do not attribute any to a louse.

(Beraisa #1): If a woman found a Kesem less than the size of three (big) Grisim, each larger than a regular bean, she is not concerned (for Zivah).

R. Yehudah ben Agra says in the name of R. Yosi, she is concerned.

Rebbi: R. Yehudah ben Agra's opinion is Nir'eh (I agree with it) when she did not check. I agree with Chachamim when she checked.

(Rabanan): The case is, she did not check her garment (until the third day, and found two Grisim), and she checked herself, but only during Bein ha'Shemashos (at the ends of days one and two) of R. Yehudah, but not during Bein ha'Shemashos of R. Yosi;

The first Tana holds (like R. Yehudah) that Bein ha'Shemashos of R. Yosi is surely night. Since she did not see during Bein ha'Shemashos (i.e. of R. Yehudah) we are concerned only for two sightings;

R. Yosi holds that since she did not check during (his) Bein ha'Shemashos, perhaps one of the sightings was then (and it counts like two), and she saw three sightings in all.

58b (Beraisa - R. Chanina ben Antignos): She may be Toleh (attribute) up to a Gris of blood on a louse, and even if she did not kill a louse.

(Rav Huna): She may be Toleh up to a Gris, but not a full Gris.

(Rav Chisda): She may be Toleh a full Gris, but not more.

Rav Chisda agrees that normally, we are stringent about the exact Shi'ur. Kesamim are different due to R. Avahu's teaching;

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

Rishonim

Rambam (Hilchos Isurei Bi'ah 9:6): A Kesem on her garment is Tamei only if it is more than a Gris. If it is less than this, she is Tehorah.

Rosh (8:6): The Halachah follows Rav Chisda.

Tosfos (52b DH Shalosh): Why did the Gemara say that two and a half Grisim were due to sightings on two days, and the additional half-Gris was from a louse? We could say that all three Grisim were from two days, like we said about less than three Grisim! Maharam said that if we would not be Toleh on a louse, and we would say that every Kesem was from her body, we would not be lenient to say that she saw them on two days, since there is a Shi'ur that can be divided into three sightings. Therefore, we suggested that two and a half Grisim were due to sightings on two days. Even though it is more than two Grisim, since it is less than three, we are lenient to be Toleh the extra to those two sightings, and not to a third sighting. Similarly, when there are three Grisim and three excesses (above a Gris), we should be Toleh two and a half to sightings on two days, and the rest to a louse!

Tosfos (Shabbos 27a DH she'Chen): Our Gemara connotes that a lentil is less than a Gris. In Pesachim (67b) we say that semen that fills the opening of the Ever is at least a Gris, and this is less than the Shi'ur Tum'ah of Sheratzim, which is a lentil! However, we can say that if a lentil is thick, when it is flattened out, it is wider than a Gris.

Poskim

Shulchan Aruch (YD 190:5): Chachamim decreed about a Kesem only if it is bigger than a Gris.

Beis Yosef (DH u'Mah she'Chasav Tolin): Even after R. Zeira's stringency (to require seven clean days after any sighting), we are not concerned for a Kesem less than a Gris. His stringency applies only to one who sees blood from her body, which is mid'Oraisa.

Sidrei Taharah (Sa'if 36, Sa'if Katan 61, DH Mihu Yesh, cited in R. Akiva Eiger): If the blood is thick in one place, and if we would flatten it, it would be a Gris, must we be concerned for this? It seems that we judge based on how it is in front of us. Conversely, if we see a Kesem that is primarily less than a Gris, but it spread and dirtied more than a Gris of the garment, she is Temei'ah. Chachamim did not distinguish. If not, if a woman saw a Kesem less than a Gris and a Chacham permitted her, and after Bi'ah the Kesem spread, e.g. due to sweat, we must forbid her. This is not "paths of pleasantness"! I brought similarly from Chacham Tzvi. However, it depends on the Rav's judgment. If the blood is very gathered in one place, we judge it as if it spread, and there is more than a Gris, so she is Temei'ah. See Tosfos (Shabbos 27a).

Pischei Teshuvah (9): The Yere'im (192) says that we are not experts about the size of a Gris, therefore we must be careful about Kesamim based on our judgment of whether a louse could have so much blood. Why didn't the Poskim bring his opinion? Besamim Rosh (135) said in his name that we are not experts about the Shi'ur, and when we are unsure if the blood is from a louse, we are not Toleh on it. I say that it is because Chazal did not specify the thickness of the blood. If there is actual blood, we estimate like this (if we are sure that it is from a louse). If the sheet is merely stained, we estimate based on a Gris. With blood the size of a Gris, one can color the entire garment! My Rebbeyim estimate with a Gris, without concern. See Sidrei Taharah.

Shulchan Aruch (ibid.): A Gris is nine lentils (Rema - three by three), and a lentil is four hairs (Rema - [so a Gris is] 36 hairs the way they are spaced on a person's body).

Beis Yosef (DH Lo): The Rashba, Rambam, and Rosh rule like Rav Chisda. Sefer ha'Terumos rules like Rav Huna; Semag refuted him. Nega'im 6:1 says that a Gris is nine lentils.

Shach (7): It is six hairs long and six hairs wide.

Pischei Teshuvah (11): The Darchei Moshe (6) said that we measure based on hairs in the head. Meir Nesivim (1:9) says that what he wrote in the Rema is primary. He normally estimates based on hairs between the wrist and elbow.

Shulchan Aruch (ibid.): If it is smaller than this, we are Toleh that it is blood of a louse, even though she did not kill one.

R. Akiva Eiger: Ba'alei ha'Nefesh says that if it is black, she is Temei'ah even if it is less than a Gris.

Pischei Teshuvah (12): Meil Tzedakah (Sof 20) said that some say that we should be Metamei a black Kesem less than a Gris, for it cannot be from a louse. (Its blood is red.) If so, the Poskim should have mentioned this! Sidrei Taharah said that he overlooked the Ra'avad, who says that she is Toleh only a red Kesem. However, in places where lice make black spots, and sheets always have such spots, surely we are Toleh on them. The Rav uses his judgment.

Shulchan Aruch (ibid.): If it is this Shi'ur, we are not Toleh on a louse, whether it is square or long.

Gra (15 Likut): We distinguish between round and long only when she wiped herself, for then (if it is from her) the blood would be long.

Shulchan Aruch (ibid.): If she found a Gris bigger than this Shi'ur, we gauge based on it.

Beis Yosef (DH u'Mah she'Chasav b'Shem): The Ra'avad says that since Chazal did not mention the bean of a particular place, or an average bean, we estimate using any bean we find, even if it is big, for the Gris of Kesamim is to be lenient.

Prishah (9, citing Maharshal): Chachamim had a tradition that (blood from) killing a louse is always like a Gris. If a bigger bean is found, there are also bigger lice.

Taz (7): It is possible that there are bigger lice (that have more than a Gris of blood).

Pleisi (7): Chazal fixed the Shi'ur to be a Gris. Even if we find bigger lentils, the Shi'ur Kesem does not change. We are unsure what are called average lentils. Maharatz gave a Shi'ur in a Teshuvah (Meil Tzedakah). I accept it only if he had a tradition for it.

Pischei Teshuvah (10): The Chasam Sofer (150, 182) toiled and verified Meil Tzedakah's Shi'ur. Even though it is much bigger than the (blood from) lice nowadays, the decree of Kesamim was a mere stringency for Taharos. Chachamim did not distinguish, and forbade her also to her husband. It would be ludicrous to say that she is Temei'ah for Taharos, and Tehorah for her husband! Since Chachamim voted about this, even though the concern for Taharos lapsed, the decree remains. Therefore, we forbid her only based on the Shi'ur from that time, when big lice were common.