Siman 486

The measure of a kezayit – we learned in the seventeenth chapter of Kelim that it is neither large nor small but medium, see there. And the dispute between Rashi and Tosafot and the Rambam is known: for Rashi and Tosafot hold that a kezayit is like half an egg, and the Rambam holds it is like a third of an egg. Therefore, regarding matzah, which is a Torah commandment – it is obvious that one should follow the stringent view and eat like half an egg. And maror, which is rabbinic – one may follow the lenient view. And regarding the final blessing and Birkat HaMazon – the rule of doubt in blessings is to be lenient, and one should not bless until he eats like half an egg. And a person should always see to remove himself from doubt. And even regarding rabbinic measures – it is proper for one to whom the mitzvah is dear to be meticulous and to fulfill it according to all opinions. And we have already explained above in siman 472, section 12, that there are some of the great authorities who wish to double the measures now, and that our opinion does not incline so. And we explained this also in siman 168 and in Yoreh Deah siman 324. And one who is stringent should be stringent for himself and not to instruct others, see there.

One must be careful when measuring a kezayit of maror with the vegetables through which one fulfills the obligation of maror, that he should compress the empty space between the leaves; and he should see to it that in the leaves themselves there should be a kezayit, and not in the air between them. And so too if there is a hollow space in the matzah – the hollow space does not join to the kezayit. However, when there is no hollow space, but it is merely soft and made like a sponge – he does not need to compress it. And it is a mitzvah to eat the entire kezayit of maror together, and not little by little. But one for whom it is difficult – he may eat it little by little, provided that he does not delay more than the time of achilat pras. And previously in siman 475, section 4, we explained that we have not found among the early authorities those who required eating the entire kezayit as one, but rather that he should eat without interruption, see there. And those who eat chrain for maror – they should dull the bitterness a little, but not a lot until no bitterness remains in it at all, for if so, he does not fulfill the obligation of maror with it. Rather, he should dull it a little.