There is a Misva to eat Massa at three points during the Seder: "Mosi -Massa," "Korech" and the Afikoman. For Mosi-Massa, the Shulhan Aruch states that one should eat two Kezaitim. The Mishna Berura explains that eating a second Kezayit solves a Halachic dilemma with regard to the Beracha "Al Achilat Massa." This Beracha is recited while holding the top, whole Massa and the middle, broken Massa, and there is a question whether it refers to the top Massa or the middle one. Therefore, one should eat a Kezayit from both.
Hacham Ben Sion and Hacham Ovadia point out that according to this reasoning, participants in the Seder who do not have a Seder Plate with three Massot in front of them and eat from the box, would not be required to eat the extra Kezayit. Nevertheless, it is preferable that all participants eat two Kezaitim at this stage.
All authorities agree that the amount of Massa eaten for the Korech sandwich is one Kezayit.
The Afikoman is eaten to commemorate the Misva of eating a Kezayit from the Korban Pesah. Therefore, one must eat at least one Kezayit of Massa for the Afikoman. The Bayit Hadash (Rav Yoel Sirkis, Poland, 1561-1640) cites a custom to eat an additional Kezayit of Massa for the Afikoman, corresponding to the Kezayit of Massa that was also eaten with the Korban Pesah.
Accordingly, the total amount of Massa that should be eaten is five Kezaitim-two for Mosi-Massa, one for Korech and two for the Afikoman (2-1-2). If one has difficulty consuming this amount, he should forgo the second Kezayit of the Afikoman and eat a total of four Kezaitim (2-1-1). If this is also not possible, he should only eat one Kezayit for Mosi-Massa (1-1-1). If this is problematic, he should forgo the Korech altogether (1-0-1). If one is able to eat only one Kezayit, he should eat the Kezayit of the Afikoman.
Hacham Ben Sion writes, based on the Rambam, that any Massa eaten on the Seder night, even beyond the requisite five Kezaitim, is a Misva in its own right. One should even reduce the amount of other foods served, in order to retain an appetite for as much Massa as possible. He found a homiletic reference to this in the words of the Ma Nishtana, "Halayla HaZeh Kulo Massa,"- This night, all we eat is Massa.
SUMMARY
Ideally, one should eat at least five Kezaitim of Massa during the Seder-two for Mosi-Massa, one for Korech and two for Afikoman.
