5. The decree of the sages regarding a kesem only relates to those that are greater than the size of a gris.  The size of a gris is 9 lentils (3x3) and the size of a lentil is 4 hairs (which is 36 hairs as found on a person's body (Maharil)).  Any time that the kesem does not have this measure, we attribute the stain to louse blood, even if she has not killed a louse.  If it does have these dimensions we do not attribute the stain to a louse, no matter what shape the stain might actually be.  If one finds a gris that is larger than these dimensions, one may use it to measure against.
    

6. This minimum size of a kesem is applicable regardless of whether the kesem is on her clothing or on her skin.  There are those who say that we only apply the standard with respect to a kesem found on clothing, but a kesem on her skin in those places for which one must be suspect, there is no minimum size.
    

7. If she killed a bed-bug or smells (a dead) one, she can attribute to it a stain up to the size of a Turmos. (a bitter legume, called "Lupine" in the vernacular)
    

8. If a stain does not exceed the size of a Gris, even if there are other spots nearby that in total exceed a Gris, she is pure - we attribute each drop to a separate louse until one individual stain is of the requisite size.  There are those who say that this applies when the stain is found on her clothing, however, if it is found on her body, they are combined.
    

9. A stain that is found on her skin that is long like a strap or is made of many drops or the length of the kesem lay across her thigh or down her thigh and is opposite her vagina is impure.  We do not say that if the stain actually came from her body that it would not have that particular shape or form.

 

What's Going On - The Size of the Stain

The Meil Tzedaka and the  Chasam Sofer both maintain that the size of a gris is a circle with a diameter of 19mm, an amount that has been described to me as being somewhat smaller than an American quarter or about the size of an American nickel.  It's possible that those two estimates mean the same thing, nevertheless one has some idea of the approximate size of a gris - of a stain that would render a woman impure.

Today there is no such thing as a louse that would leave a blood stain so large.  The halachic question arises as to whether the size of an acceptable stain is defined by the louse or by the gris.

The Yereim (12th century) maintains that the rabbinic decree is based on the size of the louse, which makes some sense since we are attributing the stain to a smashed louse, so logically any stain that cannot be attributed to a louse must therefore be impure.  Several contemporary poskim agree with this view and hold that the acceptable stain size today is limited on one 4 - 5 mm in diameter.

The other side of the argument is based on the origin of the Rabbinic decree.  This decree was made primarily in connection with permitting a woman to handle taharos - food that needs to be handled in a state of purity.  As an extension of the decree that made the blood stain impure the rabbis made the woman impure and therefore forbidden to her husband.  The defining size of an impure stain for taharos was determined to be 19 mm.  Today, since we do not have taharos, the original purpose of the decree is no longer applicable.  However, since the decree extending impurity to the woman had already been made, the Rabbis were not prepared to annul the decree completely and left it in place.  Based on this rationale, we should not be "re-sizing" the measurement for its secondary or derivative application.  Chasam Sofer and most poskim hold this view, which is the accepted contemporary halacha.

We are told that dried louse blood is always red.  Back in Siman 188 we learned that black is considered an impure color because blood will dry into a black.  Based on this, the Pischei Tshuvah (12) brings the view of the Raavad that black stains, even those smaller than a gris, are considered impure because, despite their size, we cannot realistically attribute the stain to louse blood.  However, dried flea blood is apparently black and in a place where there fleas are common one can attribute the blood  stain to the flea.  He also quotes the Meil Tzedaka who disagrees and maintains that the size limitation applies regardless of color.  Rav Moshe Feinstein concurs in this view.

The Shevet HaLevi suggests that we consider being strict with black stains that exceed the stricter 5mm view.  In this circumstance, as well as others, one needs to consult one's Rav if one is in such a situation.

Seif 6 brings us to ancillary issue.  The Mechaber tells us that the rules for minimum stain sizes apply to both stains on clothing and stains on skin.  He does, however, bring a second view, a yesh omrim, that the rules only apply for stains on clothing and that any stains on skin would render her impure.  In learning Shulchan Aruch,  we have a general principle called "Stam v'yesh, halacha k'stam" - when there is a general view and a second view using the language of yesh omrim ("there are those who say"), the halacha follows the general opinion.

Seif 8 explores a case where a woman finds many small stains that when added together exceed a gris.  Everyone agrees that if she finds multiple stains on her clothing, we attribute each stain to a different louse and do not combine the stains.  We learned in Seif 6 that there are two views regarding the applicability of the gris standard.  The first is that stains on the skin must also be gris sized and the second that all skin stains render a woman impure.  According to this latter view, the discussion about combining stains is meaningless since a stain of any size renders her impure.  However according to the first (and mainstream) opinion there is an argument as to whether skin stains and combined to compare to the standard or not.  The Shulchan Aruch brings two views, using the form of Stam and Yesh omrim that he used in Seif 6.  Although we learned that we normally hold "stam v'yesh halacha k'stam", this case appears to be an exception.  Most Rishonim according to the Sidrei Taharah, agree with the second view - that stains found on a woman's body are combined to determine her state of impurity.

To complicate matters more, the halacha of combining the stains only applies when the stains are exclusively on her body and not on her clothing as well.