Rav says that to receive malkus for eating sheratzim, one must eat a kezayis. Since the Torah uses the term "achila" it follows the rule of all issurim that malkus requires a kezayis. The gemara challenges this from a braisa that compares the issur achila to tu'mah which is b'kadasha (lentil), and concludes that when one eats a live sheretz the shiur is kezayis, but a dead one on which there is tu'mah for the size of a lentil, one would also receive malkus for consuming the size of a lentil. This distinction is paskened in the Rambam, but not explicitly paskened in shulchan aruch since even a chatzi shiur is forbidden, and the only application is for malkus.
In truth, this issue is paskened in shulchan aruch and even has a practical application. In Y.D. 104:1, the Shulchan Aruch discusses עכברא בשכרא, a rodent that falls into beer. The Shulchan Aruch writes that even if there is 60x to be mevatel the rodent, if there are pieces of it in the food that it fell into so that it can be felt upon chewing, although it is unable to be separated out, it makes the entire food forbidden. There is a big dispute between the Taz and Shach as to the rationale for this prohibition. The Taz understands this to be a rule with all non-kosher items that fall into a food from which they cannot be separated but are distinguishable when eating it, that they aren't batul. It wouldn't matter how small the pieces are, they wouldn't be batul (the Taz continues to qualify this - we are only concerned if there is a possibility that one will be eating the issur standing alone without the mixture of the kosher food). However, the Shach understands that this is an issue specific to one of the 8 sheratzim that the Torah forbids. The nature of this halacha is that since the shiur for malkus is k'adasha, the size of a lentil, we are extra strict about allowing it to be eaten through bitul. Since there is malkus for the size of a lentil we treat it like a בריה and don't allow it to be batul. Therefore, according to the shach if the pieces were smaller than the size of a lentil, or by other issurim even if the pieces were larger than a lentil, it would be batul and permissible.
This issue may really be a machlokes between rashi and the Ran. Rashi explained that the stringency of the sheretz has something to do with the shiur of malkus being the size of a lentil, but the Ran asks what do we need that for, just say it is because of חצי שיעור אסור מן התורה. The Shach explained that by other issurim we wouldn't be machmir since they are nosein ta'am l'fgam and the actual issur is batul, it is only for a sheretz where the shiur is k'adasha that we are machmir. The Mateh Yehonassan suggests that chatzi shiur that falls into a mixture no longer retains it's status of issur, unless one could consume a kezayis within an achilas p'ras. Therefore, issurim for which one can eat a shiur is not going to be batul even if there is less that a kezayis in an achilas pras. That is why we are machmir for a sheretz because even if there is less than a kezayis in achilas pras, it would still retain its status of issur so long as one can eat a lentil size of it within an achilas p'ras (it would be like any other issur in which there was a kezayis in an achilas pras).
