(Study of the Shulhan Aruch together with the rulings of Hacham Bension Abba Shaul)
The Shulhan Aruch rules that at "Mosi Masa" during the Seder, one eats a Ke’zayit from the top Masa, and a second Ke’zayit from the second Masa, which is the broken piece. The Peri Hadash (Rav Hizkiya Da Silva, 1659-1698) explained that this is done because it is uncertain whether one should eat from the top Masa or from the middle Masa. In order to satisfy both opinions, one should eat a Ke’zayit from both Masot.
Hacham Bension notes that according to this explanation, it would seem proper for every person at the Seder to have two Masot in front of him, so they can all fulfill this Halacha. In fact, since Kabbalistic tradition teaches that the special foods should be taken directly from the Ke’ara (Seder plate), everyone at the Seder should, seemingly, have his own Ke’ara. Hacham Bension observes that some people indeed follow this practice, and this is admirable, though practically speaking, this is often very difficult, particularly when a large number of people are present at the Seder. Therefore, the more prevalent practice is that only the one leading the Seder has a Ke’ara and the three Masot. After reciting the Berachot over the Masa, he eats a Ke’zayit from the top Masa and the middle Masa, and then distributes pieces of these two Masot to everyone else at the Seder. They should supplement this Masa with additional Masa taken from the box on the table.
Hacham Bension explains that in principle, it suffices to eat one Ke’zayit of Masa for "Mosi Masa," though in practice, it is customary to eat two Ke’zetim. The reason, he writes, is that according to some opinions, a Ke’zayit is twice the volume of modern-day olives, and when it comes to the Misva of eating Masa, which constitutes a Torah obligation, we should follow this stringent opinion. Therefore, one should eat two Ke’zetim for "Mosi Masa." One Ke’zayit is approximately 20 grams of Masa, and so one should eat 40 grams for "Mosi Masa."
Preferably, as numerous Poskim write, one should endeavor to eat the entire Ke’zayit all at once. This means that one places a Ke’zayit in his mouth, chews it very well, and then swallows it all together. Several Poskim, including the Hayeh Adam (Rav Abraham Danzig of Vilna, 1748-1820), as well as the Ben Ish Hai (Rav Yosef Haim of Baghdad, 1833-1909), in Parashat Sav (24), write that one should ideally eat both Ke’zetim all at once. However, if one finds this difficult, then he can eat one Ke’zayit at once, and then another Ke’zayit at once. If even this is too difficult, or if one is afraid that he will be ridiculed by others at the Seder, then he can eat the two Ke’zetim normally. He should try to eat the full quantity of Masa within a period of four minutes, but if he cannot, then he must at least eat the full amount within nine minutes. One must ensure not to talk at all until completing the full quantity of Masa that is required.
The Misva of Marror is fulfilled by eating a Ke’zayit of lettuce. Hacham Bension maintains that one should preferably eat 30 grams of lettuce for the Misva.
According to the Shulhan Aruch, one should submerge the entire Ke’zayit of Marror in Haroset before eating it. The Peri Hadash observed the custom to dip only a part of the lettuce in Haroset, but Hacham Bension writes that one should follow the Shulhan Aruch’s position and submerge the entire Ke’zayit in Haroset. One should then immediately remove the lettuce from the Haroset, as otherwise the bitterness of the lettuce will be lost. He then shakes off the excess Haroset, recites the Beracha, and eats the Marror.
For Korech, one eats one Ke’zayit of Masa – approximately 20 grams – and a Ke’zayit of Marror, together. For Korech, it is customary to dip only part of the Marror in the Haroset. One does not need then to shake off the excess Haroset.
One should not eat roasted meat at the Seder. If the Zeroa on the Seder plate had been boiled, and not only roasted, it may be eaten at the Seder. If it was only roasted, then it should not be eaten.
During the meal, one must ensure not to eat so much that he will have no appetite for the Afikoman. If one is full at the end of the meal, to the point where he does not feel like eating at all, then his eating of the Afikoman will be considered "Achila Gasa" (overindulgent eating), which does not constitute a Halachic act eating, and he will thus not fulfill the obligation of eating the Afikoman. Moreover, Hacham Bension infers from the Rambam’s formulation of the Misva of Masa that one fulfills a Misva every time he eats Masa on the night of the Seder, even beyond the Masa that is strictly required to be eaten. Therefore, instead of indulging in other foods, it is proper to eat additional Masa at the Seder. It should also be noted that the Zohar refers to Masa as "Michla De’asvata" – "food of healing," meaning, Masa has a beneficial, "healing" impact upon the soul.
If one prepared to recite Birkat Ha’mazon and remembered that he had not eaten the Afikoman, then he eats the Afikoman at that point, and does not recite a new Beracha of "Ha’mosi." Even if one had announced "Hab Lan Ve’nibrich," introducing Birkat Ha’mazon, he does not recite a new Beracha over the Afikoman. Normally, once this announcement is made, one is assumed to have diverted his attention from eating, such that if he then changes his mind and decides to eat, he must recite a new Beracha. At the Seder, however, this Halacha does not apply. The reason is because we sit at the Seder as G-d’s guests, and a guest, who is dependent upon his host, always anticipates that more food might be served, such that he never actually diverts his attention from eating. Therefore, if one prepared to recite Birkat Ha’mazon but then realized he needs to eat the Afikoman, he does not require a new Beracha over the Afikoman.
The Shulhan Aruch rules that if one remembered only after Birkat Ha’mazon, before drinking the third cup of wine, that he had not eaten the Afikoman, then he washes Netilat Yadayim, recites Ha’mosi, eats the Afikoman, and then recites Birkat Ha’mazon again over the third cup of wine. If one remembered only after drinking the third cup, then he should not eat Afikoman, but should instead rely on the Masa he ate during the meal, which may retroactively be considered his Afikoman. However, if the Masa he had eaten during the meal was not Masa Shemura, then he must wash and eat the Afikoman. He then recites Birkat Ha’mazon over another cup of wine.
If the Afikoman was lost, one may take other Masa as his Afikoman.
It is preferable, if one can, to eat two Ke’zetim of Masa as the Afikoman. The Afikoman commemorates the meat of the Korban Pesah, which was eaten together with Masa, and so one should preferably eat one Ke’zayit of Masa in commemoration of the Korban Pesah, and a second Ke’zayit in commemoration of the Masa with which the sacrifice was eaten. If this is difficult, however, it suffices to eat just one Ke’zayit of Masa as the Afikoman.
It turns out, then, that one should, ideally, eat five Ke’zetim of Masa at the Seder: two for "Mosi Masa," one for Korech, and two for the Afikoman.