2,106. The Requisite Size of Blood Stains
Hilchos Issurei Biah 9:5
A virgin (in the previous halacha) means a girl who has never menstruated even though she has experienced hymeneal bleeding because of marriage or uterine bleeding because of childbirth. An elderly woman means one who hasn’t menstruated for 90 days close to old age. A woman is considered elderly when she can be called an old woman without objecting. If a pregnant, nursing or elderly woman sees a stain, it’s the same as seeing blood: she isn’t rendered unclean retroactively. When it comes to a virgin who is a minor and has never menstruated, a stain is ritually clean until she has menstruated for three consecutive months.
Hilchos Issurei Biah 9:6
The difference between a stain on a woman’s skin and one on her clothing is that there’s no minimum size for a stain found on her skin but a stain on her clothing doesn’t render her ritually unclean unless it’s the size of half a Cilician bean, which is a square large enough to hold nine lentils in three rows of three. If a stain on clothing is smaller than this, the woman is ritually clean. If the stain is made up of multiple small spots, they do not combine to form a single unit. If the stain is long and thin, it combines to form a single unit.