The gemara elaborates that chazal are extremely precise in their shiurim. Tosafos comments that even when they give the shiur of mikvah as 40 seah, and say that even a "kurtav" less than 40 seah is a pasul mikvah, they don't mean to say that a half of a kurtav less than 40 seah would be kasher. Tosafos holds that even the smallest measurable amount less than 40 seah would invalidate the mikvah.
In my sefer on Mikvaos, "Mayim Rabim" (pg. 304) I mentioned this Tosafos in the context of the Rashbatz quoted in the Beis Yosef (Y.D. 201) who claims that "kurtav" is to be taken literally. As a proof to Tosafos, I pointed out that we find a similar expression by nidah that even a drop of blood like a "chardel" renders her ta'mei, and clearly chardel is not literal, but even less that a chardel would make her ta'mei.
However, I didn't mention any sources to prove the point by chardel. The Shach in Y.D. 183:3 clearly states that even less than a chardel is ta'mei. Sidrei Taharah 183:4 concurs and cites an explicit Rashi in Nidah 14b who says that even less than a chardel is ta'amei - and therefore is amazed that the shevus ya'akov seems to elaborate on this issue.
Hayotzei Midvareinu, that when our gemara says that midos chachamim are precise, they are sometimes even more precise than the exaggerated measure mentioned, and in actuality the halacha is determined by the smallest measurable amount (obviously, w/o the use of microscopic instruments).
