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 Pesaḥ. 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-khatḥila, it is good to have both of these reasons – to commemorate the korban Pesaḥ 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 Pesaḥ, 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 Pesaḥ. 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 does not disqualify 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 Pesaḥ and another fulfills the mitzva to eat matza (Baḥ). 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, in simplistic terms, a kezayit is about a third of a machine-made matza, and two kezeytim are about two thirds. This calculation is based on the stringent view (Tosafot) of the size of a kezayit. But since eating the afikoman is a rabbinic mitzva and there is disagreement about whether one must eat a second kezayit, one may fulfill the mitzva of afikoman by eating one kezayit of one third of a matza and, if necessary, one fifth of a matza. We have already learned that whoever eats the matza without interruption will certainly finish it within a shi’ur akhilat pras.
Regarding the size of a kezayit, see above section 23 and nn. 20 and 25; in cases of uncertainty 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 – but vis-à-vis rabbinic mitzvot over which no berakha is said, like eating the afikoman, one 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. When necessary, one even satisfies the requirements of the enhanced practice of eating two kezeytim with the smaller kezayit, as we have seen that according to many Ge’onim and Rishonim a kezayit is the size of our common olive, and one fifth of a matza contains 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.