The Biblical mitzvah of maror was fulfilled by eating the maror vegetables with the korban pesach, which we no longer have. The Chachamim nonetheless instituted that we continue the mitzvah even sans korban. The Rosh (perek arvei pesachim) writes that a minimum of a k’zayis (an olive-size piece) of maror must be eaten because we say a bracha of “al achilas maror”. The Sha’agas Arye raises a number of difficult questions on this explanation of the Rosh. For any mitzvah or issur, whether the term “achila” is used or not, the definition of eating is always assumed to be a k’zayis sized portion. For example, one does not violate the Biblical commandment of eating basar b’chalav unless one has eaten a k’zayis portion, even though the Torah never uses the term “achila” in the context of the issur. The fact that the bracha on maror (which the Rosh refers to) contains the term “achila” seems entirely irrelevant to consideration of the amount that must be consumed to fulfill the mitzvah. Secondly, one clearly fulfills the mitzvah of maror even if the bracha is omitted – why then should the recitation of the bracha be the criteria which defines the mitzvah itself? Finally, the Rosh simply begs the question of why the Chachamim formulated the bracha as "al achilas maror", obligating us to eat a k'zayis, instead of simply formulating the bracha as "al mitzvas maror", avoiding the term "achila" entirely.
Rav Hershel Shachter in his sefer Eretz haTzvi offers an explanation of the Rosh that is consistant with the thinking of the Brisker Rav. Since we no longer have a korban, the mitzvah of maror is a remembrance to the mitzvah that was, a type of zecher l’mikdash. As we saw yesterday, one could very well argue that a mitzvah which is celebrated as a zecher l’mikdash should not require a blessing – recall the gemara (sukkah 46) that RYb”L held that one can not say a bracha of “al netilas lulav” because taking the lulav fulfills a new mitzvah of zecher l’mikdash, not the old mitzvah of lulav, and Rabeinu Yerucham held that an independent bracha on counting weeks of sefira is not recited because counting weeks is only a zecher l’mikdash.
Furthermore, if zecher l’mikdash is indeed a new form of mitzvah, the entire tzuras hamitzvah, the format of the mitzvah’s performance, need not conform exactly to the rules that governed the old mitzvah. Perhaps this is precisely why many of the disqualifications that render an esrog unfit on the first day of Sukkos are suspended during the remaining days – it is not merely that the mitzvah is only Rabbinic in nature on those days, but that the mitzvah being fulfilled is not one of lulav and esrog but of zecher l’mikdash. The Ran writes that in cases of extreme need the mitzvah of koreich done on the seder night need not be done with matzah shemurah because it is a zecher l’mikdash (as we recite in the haggadah) - even if non-shemurah matzah is disqualified as matzah, it is not disqualified from being used as a zecher.
Is the mitzvah of maror a true zecher l’mikdash mitzvah like these other examples? This is exactly the issue the Rosh was addressing. The Rosh never meant to prove the requirement of eating a k’zayis from the text of the bracha, as the Sha'agas Arye understood. What the Rosh meant is that the very fact that a bracha of “al achilas maror” is recited proves that the mitzvah of maror is not just a fulfillment of a new concept of zecher l’mikdash, but is a fulfillment of the same old mitzvah of maror which we continue to practice even though we no longer have a korban. And since the mitzvah is not a new enactment of zecher l’mikdash, but is an extension of the same old mitzvah of maror which always existed, the tzuras hamitzvah, the format of the mitzvah in terms of what must be eaten and the quantity which must be eaten (a k’zayis) remains unchanged.