The Afikoman is eaten right before Birkat Hamazon at the Seder, and is the last food eaten at the Seder. The Shulhan Aruch rules that one must eat at least one Kezayit of Masa to fulfill the obligation of Afikoman. Some, however, have the practice of eating at least two Kezetim of Masa for the Afikoman. The Afikoman is eaten in commemoration of the Korban Pesah, and in the times of the Bet HaMikdash the meat of the Korban Pesah was eaten together with Masa. Therefore, some authorities maintained that one must eat one Kezayit of Masa to commemorate the sacrificial meat, and another Kezayit to commemorate the Masa that was eaten along with the meat of the sacrifice.
Therefore, if one is able to eat two Kezeitim for the Afikoman, it is proper to do so in order to fulfill all opinions. However, if a person feels very full and cannot eat more than one Kezayit, he should eat only one Kezayit and he thereby fulfills the Misva, in accordance with the ruling of the Shulhan Aruch.
It thus emerges that a person must eat a total of four or five Kezetim of Masa at the Seder: two for "Mosi Masa," one for Korech, and one or two for the Afikoman.
Invariably, the Masa that had been put away for the Afikoman before Maggid will not suffice to supply a Kezayit (or two Kezetim) for everyone at the Seder. Therefore, one should take Masa from the box at the table so he could fulfill the Misva properly. The one leading the Seder should ensure that there is enough Masa for everyone to receive one Kezayit for Afikoman, or two Kezetim for those who wish to follow the stringent position.
The Kezayit of the Afikoman should be eaten within a period of four minutes, or at most within seven minutes.
One must eat the Afikoman while leaning ("Heseba"). If one ate the Afikoman without leaning, he should eat another Kezayit while leaning. If, however, he feels too full and eating more Masa would constitute "Achila Gasa" (vulgar overeating), then he may rely on the view which does not require leaning for the Afikoman.
Both men and women are required to eat the Afikoman, with "Heseba."
In the Haggada we read the famous Halacha of "En Maftirin Ahar Ha’pesah Afikoman," which means that one may not eat anything after he eats the Afikoman. Dessert should be served before the Afikoman, and not afterward. It is also forbidden to drink after eating the Afikoman, with the obvious exception of the remaining two cups of wine, which are drunk after Birkat Hamazon and after Hallel. Furthermore, it is permissible to drink water after the Afikoman, and one who wishes to stay up after the Seder to learn or sing may drink some tea or coffee to help him stay awake.
Summary: At the Seder, one must eat for the Afikoman at least one Kezayit of Masa, within four minutes (or at most seven minutes), while leaning. Some have the custom to eat two Kezetim. One who did not lean while eating the Afikoman should eat another Kezayit while leaning, unless he is too full to eat more Masa. One may not eat or drink anything on the night of Pesah after the Afikoman, except for the two remaining cups of wine, and water. One may also drink tea or coffee to help him stay awake.