Shalom Alecha HaRav Kornfeld, Shlita!
In your insights on 79b, it is noted that the poskim do not address Rav Papa's query of whether or not toinclude the pit computing the shiur of a koseves because the gemara concludes that the size of a koseves is less than a kibeitzah. Thus, the poskim are content with ruling that a koseves is "a bit smaller than a kibeitzah". My question is that this does not seem to solve the problem. The poskim do not tell us how much smaller a koseves is than a kibeitzah. Thus, we still do not know the exact size of a koseves!
The Poskim, as quoted in the Insights, say that a Koseves is a little less than a k'Beitzah. Although we are not told of the exact size of a Koseves, that is a sufficient approximation of the size for the purposes for which we need to know the Shi'ur of the Isur of Achilah on Yom Kipur.
Indeed, Rav Papa's question, according to none of the explanations of the Gemara, was what is the size of a Koseves, but rather whether the "Koseves and the pit" mentioned in the Mishnah includes an additional pit, or includes the space between the flesh and the pit. Each of those sizes are known, and the question was only to which one is the Mishnah referring.