As we have seen, according to most authorities the afikoman commemorates the Paschal sacrifice, which was eaten at the end of the meal (Ha-ma’or, Ramban, Or Zaru’a, Rosh, etc.). However, according to several major Rishonim (Rashi, Rashbam), the afikoman is actually the fulfillment of the fundamental mitzva of eating matza, which, they explain, must be eaten at the end of the meal along with the korban Pesaĥ. Since the Paschal sacrifice had to be eaten while satisfied, so must the matza. And even though we recite the berakha over the matza at the beginning of the meal, in their opinion the basic intention to fulfill the mitzva to eat matza must be at the end of the meal, when the afikoman is eaten.
Le-khatĥila, it is good to have both of these reasons – to commemorate the korban Pesaĥ and to fulfill the mitzva of eating matza – in mind while eating the afikoman. According to both opinions, the afikoman must be eaten while reclining to the left. However, if one forgets to recline, there is a difference between them: according to the opinion that the afikoman commemorates the korban Pesaĥ, one need not eat another afikoman while reclining. According to the opinion that the afikoman fulfills the mitzva of eating matza, reclining is a sine qua non. Therefore, if one forgot to recline while eating the afikoman, he should preferably eat a second one while reclining. However, if one is full and will have a hard time eating another kezayit of matza, he need not eat the afikoman a second time; he may rely on the mainstream opinion that the afikoman commemorates the korban Pesaĥ. Moreover, even according to Rashbam, since one intended to fulfill the mitzva of eating matza at the beginning of the meal, he has fulfilled his obligation even though, for Rashbam, that was not the proper time to have such intention. Therefore, failing to recline while eating the afikoman no longer prevents the fulfillment of the mitzva.
Some have a custom to eat two kezeytim of the afikoman, either because they want to show how desirable the matza is and thus eat a sizable portion to become fully satiated (Maharil), or because they want to allude to both reasons for eating the afikoman: one kezayit commemorates the korban Pesaĥ and another fulfills the mitzva to eat matza (Baĥ). However, this is not obligatory, and if one does not want to eat two kezeytim, he may eat just one and still keep both reasons in mind.
We have seen that, simply stated, a kezayit is about a third of a machine-made matza, and two kezeytim are about two-thirds. This is based on Tosafot’s stringent opinion about the size of a kezayit. In light of the fact that the afikoman is a rabbinic mitzva and that consumption of two kezeytim is subject to disagreement, one may certainly fulfill the mitzva of afikoman with a single kezayit of one third of a matza and, if necessary, with one fifth of a matza (based on Rambam’s smaller kezayit). And we have already learned that whoever eats the matza without interruption will certainly finish it within a shi’ur akhilat pras.
. SA states that one must eat a kezayit of matza for afikoman. Darkhei Moshe quotes Maharil that one must eat two kezeytim for afikoman, and similarly, MB states in 477:1 “le-khatĥila, one should take two kezeytim.” This is also the opinion of Kaf Ha-ĥayim 477:1. Sefer Ha-ĥinukh §21 implies that one can fulfill his obligation with less than a kezayit, and AHS 477:3 states that in an extreme situation one may use less than a kezayit, since the afikoman is only intended to commemorate the korban Pesaĥ. Responsa Ĥazon Ovadia §44 expands on this topic and presents other opinions, concluding that the mitzva is to eat one kezayit, although one who wishes to be stringent should follow MB. Several Aĥaronim write that the one may have in mind to fulfill both intentions with just one kezayit.
Regarding the size of a kezayit, see above section 23 and nn. 20 and 25, where we wrote that in unclear situations regarding Torah commandments or mitzvot that require berakhot we follow the stringent view of Tosafot – that a kezayit is the volume of half an egg. On the other hand, in unclear situations vis-à-vis rabbinic mitzvot with no berakhot we may follow the lenient view of Rambam – that a kezayit is less than one third of an egg, or c. one fifth of a machine matza. Therefore, regarding the rabbinic mitzva of eating the afikoman, for which only a minority of poskim maintain that one must eat two kezeytim, one may certainly fulfill the stringent opinion that eating two kezeytim of afikoman can be fulfilled by eating one fifth of a matza, since according to many Ge’onim and Rishonim a kezayit is the size of a common olive. One fifth of a matza thus constitutes several kezeytim by this standard. Regarding shi’ur akhilat pras, see above section 25 and n. 22.
See section 25 for a discussion of the measurement of akhilat pras; in general, the most stringent opinion is that the time of akhilat pras is four minutes, and the median opinion is that it is between six and seven minutes. Since afikoman is a rabbinical decree and no berakha is recited on it, one can eat the afikoman in 6-7 minutes, and even if he ate it in nine minutes, he fulfills his obligation, since we follow the lenient view in an unclear situation vis-à-vis rabbinic mitzvot with no berakhot.