Yu ed to eat one Kazayit by Motzi Matzah, one Kazayit by Korech and one by Afikomen. However, in order to make a bracha of Al Achilat Matzah you need a bigger amount.
Explanation:
The shiur of Matzah by Motzi Matzah:
Tur 475: Wash your hands and take the plate with matzot that one whole is on top, and the broken is on the bottom. Say Hamotzi on the whole ne and brake it and say Al Achilat Matzah on the broken one and eat a Kazayit from each.
The Rambam (Chametz U’Matzah 8:8) :Nowadays when we don’t have Korban Pesach after you say Hamotzi you say Al Achilat Matzah and dip the matza in Charoset and eat it.
The Rambam does not specify if one needs to eat one or two Kazaitim. The fact that he leaves it out implies that you only need eat one Kazayit.
The Shulchan Aruch (475:1) writes that you need to eat a Kazayit from each like the Tur.
The Bach and the Prisha ask why do we need wo Kazeitim, or Hamotzi one only needs to eat a little bit. Only for the bracha on the mitzvah of Achilat matzah do you need a Kazayit to be called an Achilah-eating.
The Bach answers the Tur and Shulchan Aruch are only saying ideally you need a Kazayit for Hamotzi. Really you don’t need a Kazayit for hamotzi but since you have a Kazayit in front of you should eat a Kazayit. The Magen Avraham follows the Bach that two kazeitim is only lechatchila.
The Prisha writes that the Tur was unsure which matza to say Al Achilat Matzah on, so therefore you say the brachot on both and eat a Kazayit from both.
So rules the Mishna Berura that since we are not sure which matzah Al Achilat matzah refers to so we need to eat a Kazayit from each one.
R’ Ovadia Yosef (Chazon Ovadia Pesach pg. 65) writes that you break both matzot together and eat a Kazayit from each, however if one ate only a Kazayit from either one he fulfils his obligation. If one has a hard time with two you can eat one.
For Korech:
The Gemara in Pesachim 115a writes: They said that Hillel would wrap everything together and eat them as it says “Al Matzot Umerorim Yochluhu. R’ Yochanan says Hillel’s colleagues argues with hm… Rav Ashi says might think you only fulfill your obligation by wrapping them together like Hillel, the Mishna says Al Matzot Umarorim, even if they are by themselves. Nowadays since we don’t have e clear ruling like Hillel or not so we say Al Achilat Matzah and eat, and then Al Achilat Maror and eat and then we eat Matzah and Maror together and say Zecher Lamikdash K’hillel.
So rules the Shulchan Aruch 475:1.
It seems that the shiur is the same for Korech and Matzah and Maror, a Kazayit.
Afikomen:
The Rambam (Chametz uMatzah 8:9) : At the end one eats from the meat of the korban Pesach even just a Kazayit and doesn’t eat anything afterwards and nowadays we eat a Kazayit of matzah and doesn’t eat anything afterwards.
So rules the Shulchan Aruch (477:1) that one Kazayit is sufficient: after the end of the seuda everyone eta a Kazayit from the Matzah that we put aside under the napkin.
The Magen Avraham quotes the Maharil that you really need to eat two Kazayit one for the remembrance of the Pesach and one for the Matzah that was eaten with it. The Taz also quotes the Maharil but says one needs two because the mitzvah is very loved and special so you need two which is a Bietzah. So ruled the chok Yaakov and the Mishna Berura.
R’ Ovadia Yosef writes that there are those who are stringent on eating two kzaiitim one for Pesach and one for the matzah, but the man halacha is that one Kazayit is sufficient so rule most Rishonim and the Shulchan Aruch. One who eats two kazeitim should be blessed but one who eats one Kazayit is fine which is a third of a beitzah.
Therefore, the least amount of Kazayit you can eat is three:
One fore Motzi Matzah, one for Korech, one for Afikomen. If one can eat four should eat two by Motzi Matzah. If one can eat five then one should eat two for Motzi Matzah , one for Korech and two for Afikomen.
