The Mishna has three opinions for how long a hair must be to be considered a hair. The first opinion is that it must be long enough to bend the tip back to the root, the second opinion is that it must be long enough to catch a nail when it runs over it, and the third is that it must be able to be cut with a scissors. Rav Chisda says in the name of Mar Ukvah that we pasken like all of them to be machmir. Tosafos explains that she can't do chalitza until she has 2 hairs that meet all the requirements, and she couldn't do mi'un even if she has hairs that meet the minimum requirement (Tosafos says in the name of Rabbeinu Tam that since the hairs can be anywhere and don't need to be adjacent to one another, and they can even be follicles without actual hairs, we are machmir not to allow mi'un as soon as she is 12 years old even if we don't find any hairs).
The Rambam (Hil. Ishus 2:16) writes that when a boy or girl have 2 sa'aros, from the stage of being able to be cut with a scissors until the stage of being able to bend it back to its root, we are machmir to consider them adults, but not lenient.
Rav Chaim (Brisker) questions why the Rambam omits the middle shiur of כדי לקרוץ בצפורן. Although generally being machmir for the largest and smallest is sufficient, there would still be a point to mention the middle shiur. For example, in a case where she married herself off after stage 1, stage 2 and stage 3 to 3 different people, we shouldn't say that it is sufficient to receive a gett from 1 and 3, rather she would also need a gett from 2 because she may have transformed into a gedola at that stage? Furthermore, Rav Chaim asks that the Rambam in the context of Para Aduma in discussing the halacha of 2 black hairs invalidating the parah aduma, writes that you can trim the hair down to the shiur of "being cut by a scissors" and if what remains is red and not black, the para aduma is kasher. The kesef mishna asks why is the Rambam lenient for the small shiur of being cut by a scissors, we should be machmir for the larger shiur of bending the head back to the root so that the hairs would be considered black and passul the para aduma?
Rav Chaim writes that the halacha of כדברי כולן להחמיר is only for halachos in the hair themselves. For example, when there is a halacha that through these hair we would consider him/her to be a gadol, we are machmir in both directions for the shiur of what qualifies as a hair. Since the halacha demands "2 hairs" we are machmir that it does't qualify until it is large enough to bend back to its root. However, in the context of para aduma the halacha of the hairs being red isn't a halacha in the actual hairs. The halacha doesn't demand a hair that has red roots. Being that the halacha of red hairs doesn't require the hair to have a status of a hair in halacha, we can follow the minimum din that would allow us to consider the hair to be a hair - being able to be cut with a scissors.
However, Rav Chaim points out that his approach is only plausible if we are to say that the din of הלכה כדברי כולן להחמיר is not a halacha resulting from a safeik of not knowing who to pasken like. Rather, it is a halacha that is said בתורת ודאי. Meaning, all agree that hairs that meet the minimum requirement has status of a hair, just that the halacha demands that to consider her a gedola the hairs must meet the maximum requirement. If it were simply a halacha resulting from not knowing who to pasken like, it would be obvious that we couldn't be lenient until we were sure she was an adult. Rather, there are 3 legitimate shiurim said by hairs, the smallest allowing the hair to have status of hair, and the largest shiur invalidating the smaller shiur and establishing a larger shiur. Based on this approach, the halacha only considered the largest and smallest to be legitimate shiurim of hair for which we must be concerned. The middle shiur is not part of this halacha and therefore the rambam omits it from his halacha.