Negaim, Chapter 6, Mishnah 2. Most of this chapter is devoted to the various configurations of a michyah, a patch of healthy flesh, found inside a baheres. Before dealing with what sits inside the nega, however, the Mishnah first pins down the size of the nega itself: the shiur kagris, the measure of a gris, which we have encountered many times already. Here the Mishnah defines it with great precision.
The opening words are "Gufa shel baheres", literally the baheres itself, meaning the actual mass of the blemish. Keep in mind that baheres here is not restricted to one particular shade: it serves as a general term for all four degrees of whiteness enumerated in the first perek, and also for the four cases in which red is intermingled with those whites. Regardless of which shade we are dealing with, the mark must cover a set amount of skin.
Kigris hakilki meruba
That measure is "kigris hakilki meruba". The Rav explains that a gris is a chatzi pul, half of a bean, that is, a bean that has been split in two. The root gares carries the sense of cutting or grinding, and it is the same root we use in learning: the girsa of the Gemara is what has been ground into a person's mind and fixed in his intellect.
Kilki is a place name. The commentators note that the beans grown there were unusually large, and the Peirush HaMishnayos cites the Tosefta that the gris the Mishnah is speaking of is wider than the breadth of a person's thumb. This is a fairly substantial measure.
To this the Mishnah appends the word "meruba", meaning square. Now, no bean is actually square; beans are oval, elliptical, at times almost circular. The Rav therefore explains that we take the surface area of the bean and reconfigure it as a square (others understand the square as one whose side matches the bean's length). It is that square which has to have room inside the baheres. When the square of a gris can be set down within the borders of the discoloration, the nega is of a size that can carry tumah. When there is no way to fit that square inside it, the mark falls short of the required measure and cannot make a person tamei.
Nine lentils and thirty six hairs
From here the Mishnah moves to smaller units of measurement. "Mekom hagris tesha adashos": the space occupied by a gris is equivalent to nine lentils. Arrange nine adashos alongside one another and you have the area of a gris, which is to say the smallest baheres that counts.
And the Mishnah breaks it down one step further: "mekom ha'adashah arba se'aros", the area of a single lentil is four hairs, that is, two hairs by two hairs, each set a slight distance from the next. So every adashah accounts for four hairs' worth of area.
The conclusion follows by simple arithmetic: "nimtzu shloshim v'sheish se'aros". Nine lentils times four hairs each yields thirty six. Within the space of a gris there are thirty six hairs, with a small gap between one hair and the next. That is the precise minimum area of a baheres.