How much food must a person eat to require the recitation of a "Beracha Aharona" after eating?
With regard to the "Beracha Rishona" the Beracha recited before eating the Sages did not designate any minimum Shiur (quantity). Even if a person sips a drop of water or partakes of a tiny morsel of food, Halacha requires that he first recite a Beracha. When it comes, however, to the Beracha Aharona, the Sages indeed limited this obligation to the consumption of a minimum amount. The Shulhan Aruch (Orah Haim 210:1; listen to audio recording for precise citation) rules that a person recites a Beracha Aharona after eating food only if he ate at least a "Ke'zayit," or an olive's worth of food. One who eats less than a Ke'zayit must recite a Beracha before eating, but does not recite a Beracha Aharona after eating.
How large is a Ke'zayit?
Amidst the presentation of the laws of Pesah, the Shulhan Aruch (Orah Haim 486) cites two views among the Rishonim (Medieval Halachic scholars) in defining a Ke'zayit. Whereas some Rishonim defined a Ke'zayit as half a Ke'besa (an egg's worth), other maintained that a Ke'zayit is measured at only one-third of a Ke'besa. The Mishna Berura (commentary to the Shulhan Aruch by the Hafetz Haim, Rabbi Yisrael Kagan, 1839-1933) writes that given the principle of "Safek Berachot Le'hakel" that one should not recite a Beracha in situations of uncertainty a person who eats less than a half a Ke'besa should not recite a Beracha Aharona. Even though some views require the recitation of Beracha Aharona after eating only one-third of a Ke'besa, a person should not recite a Beracha in such a case in deference to the opposing view. This is the position of many other authorities, as well, including Hacham Ovadia Yosef, Hacham Ben Sion Abba Shaul (Jerusalem, 1923-1998), and Rabbi Moshe Halevi (Israel, 1961-2001).
The Mishna Berura then adds that one should preferably avoid this situation by ensuring never to eat food of an amount between a half-Ke'besa and one-third of a Ke'besa. One should make a point to either eat less than one-third of a Ke'besa in which case he does not recite a Beracha Aharona according to all opinions or eat at least a half-Ke'besa in which case he must recite a Beracha Aharona according to all opinions.
How is this size of a half-Ke'besa measured?
The Shulhan Aruch addresses this question amidst his discussion of the laws of Halla (the piece of dough one must separate from the batter when baking), where he writes (Orah Haim 456) that the minimum quantity of dough requiring the separation of Halla is the volume of 43.5 eggs. This is measured, he adds, by placing this quantity of eggs in a container filled to the rim with water, and collecting the water that is displaced as a result. This amount of water represents the minimum amount of dough required for the Halla obligation to take effect. Thus, at least with respect to the obligation of Halla, a Ke'besa is measured by volume, and not by weight. Many authorities, including Hacham Ovadia Yosef and Rabbi Moshe Halevi (in Birkat Hashem, vol. 2, 1:6), and Hacham ben Sion Aba Shaul, apply this principle to the laws of Berachot, as well. Hence, when we speak of a half-Ke'besa as the minimum amount of food requiring the recitation of a Beracha Aharona, we refer to the volume of a half an egg, and not the weight of half an egg.
Summary: One must recite a Beracha before eating any quantity of food, even a tiny morsel. A Beracha Ahrona after eating, however, is required only if a person ate a half-Ke'besa, or the volume of half of an egg.
