The amount of a ke-zayit for rabbinic decrees is 27 cubic centimeters (1.65 cubic inches). However, according to the letter of the law one may use a smaller quantity for the ke-zayit of korekh, namely 19 or even 17 cubic centimeters (just over a cubic inch) – namely, a medium or small leaf of lettuce, because this is only a remembrance of the Temple practice, and many poskim were lenient in the size of maror (including the Chazon Ish and Rav Shlomo Zalman Auerbach).
One should eat the korekh continuously (within four minutes, but there is no need to time it with a watch).
The type of maror
There are those who wrote that, according to the Ari z”l, it is preferable to use Romaine lettuce for maror and horseradish for korekh. The Magen Avraham (473:12) wrote that the custom is to use the leaves of Romaine lettuce for maror and the stalks of Romaine lettuce for korekh.
The Taz (ibid., subsection 5), though, wrote that “there is no rhyme nor reason” for this differentiation, and those who do so “do not know their right from their left.” There are those who wrote that the “origin of this is that the seder plate of the Ari z”l had one place for “maror” and another for “chazeret,” but he did not mean that there are different things.
In practice, there is no need to differentiate between the vegetable to be used for maror and that to be used for korekh, however every person may do as he wishes, as any of these is acceptable (and according to Chazon Ish there is even an advantage in eating horseradish, and if that is so, it is possibly better, according to him, for the maror. And there are those whose custom is to eat Romaine lettuce for korekh, but to add a little horseradish).
It is better to lean when eating the korekh, for Hillel certainly ate it while leaning, given that, according to him, this is the obligatory eating of matza. If one did not lean, he has nevertheless fulfilled his obligation (Peri Chadash, 475:1; Shulchan Arukh ha-Rav, 475:20).
Adapted by Rabbi Dov Karoll from Pesach Haggadah Shirah Miriam – Haggadah MiMekorah by Rabbi Yosef Zvi Rimon, published by Mosad Harav Kook in conjunction with the Halacha Educational Center, firstname.lastname@example.org.
 According to the Sages, it would have been enough if the Torah had simply said for “they shall eat it” – yokhelu. The fact that the Torah states yokheluhu teaches us that each item can be eaten alone (Gemara and Rashbam ibid.).
 Because of this dispute, there are various opinions as to what should be recited in the haggada: according to the Taz (475:9), one should say: “so did Hillel at the time that the Temple stood: he would wrap the Pesach sacrifice, matza and maror …” However, the accepted version of the text is, “he would wrap matza and maror…” (Mishna Berura ibid., subsection 21). The Chok Yaakov (ibid., 13) explains that this latter version is based on two reasons: a) According to the Rambam, Hillel would only wrap the matza and maror together. b) As we do not eat the Pesach sacrifice nowadays, the text recited should be in keeping with what we are doing, and there is a concern that if the alternate version is recited, some might eat roasted meat along with matza and maror.
 Tosafot (ibid. s.v. ella) understood that Rabbi Yochanan’s view is that at the outset one must wrap the matza and maror together, even according to the Sages, but if one does not do so he has nevertheless fulfilled his obligation. This view requires study. See the Maharsha there. The Peri Megadim (Mishbetzot Zahav, 475:7) writes that even according to Tosafot the Sages would prefer that one eat the two separately, while if one nevertheless ate them together he still fulfilled his obligations.