838. Korech

119:6 If a person cannot chew matzah, he may soak it in water to soften it so long as it does not completely dissolve. If a person is old or ill and cannot eat matzah soaked in water, he may soak it in wine or in any other liquid. (If anyone else does this, they would not fulfill their obligation - Mishnah Brurah 461:18.) When soaking the matzah to be used for the mitzvah, one must be careful not to soak it for 24 hours, which is considered a form of cooking and would invalidate the matzah. One must likewise be careful about other things that could cause the matzah to lose its status as bread. See chapter 48:7 for more details.

119:7 Next, the leader gives each person an olive-sized portion (k'zayis) of maror. The maror is dipped in the charoses; the charoses is then shaken off so as not to affect the taste of the maror. One recites the bracha "al achilas maror," that G-d commanded us concerning the eating of maror. The maror is eaten without leaning (because it is in memory of the servitude, though if one chooses to lean, it is of no consequence - MB 475:14). One then takes a k'zayis from the lower matzah and a k'zayis of maror; it is proper to dip this also in charoses and to shake it off. The maror is placed between pieces of matzah and we recite "Thus Hillel did..." The sandwich is eaten while leaning.

The size of a k'zayis is about half an egg, though some authorities say it is a little less than one-third of an egg. Since eating maror nowadays is a rabbinic enactment, one who has trouble eating maror may rely on the latter opinion and eat only a little less than one-third of an egg's volume; he may still recite the bracha over doing so. If a person is ill and cannot eat any maror at all, he should at least chew a little so that he has the bitter taste of the maror in his mouth. This serves as a reminder of the mitzvah but he may not recite the bracha on this.