Eating the matzot on the Seder night

Shalom,
On the Seder night one must eat two kezayit of matzah. Is the obligation of these two kezayit specifically on the head of the household, or are the other members of the household also obligated to eat two kezayit? And if the other members of the household are also obligated, does everyone need to eat specifically from the matzot in the Seder plate (from the three matzot), and is it at all possible for everyone to eat a kezayit or two kezayit from the head of the household’s matzah?

Another question: the measure of two kezayit is 30 cc (each kezayit 15 cc). Does this mean that in this way one certainly fulfills the Torah obligation of eating a kezayit, and since by rabbinic law one must eat two kezayit — they established the measure according to this smaller size?

Shalom u’vracha 

A. Regarding the mitzvah of eating matzah, there is no obligation to eat two kezayit, only one kezayit. 

The one conducting the Seder, who recites the blessing over eating the matzah, should preferably eat one kezayit from the upper matzah and one kezayit from the middle one. 

The other participants should eat one kezayit, as required by law, and it is preferable that they also receive a piece from the upper and middle matzot over which the blessing was recited. 

Some have the custom that each participant, or each person above bar mitzvah age, has three matzot in front of him and recites the blessing himself. In that way, each one has a kezayit from the upper matzah and a kezayit from the middle one. 

With our matzot there are not so many kezayit, and if there are several participants at the Seder, one should make up the quantity of a kezayit from other matzot. 

B. There is a dispute regarding the size of a kezayit: some say about 27 cc, and some say about 15 cc. Since eating a kezayit of matzah is a Torah obligation, one should be stringent and follow the larger measure. However, eating two measures of a kezayit is not a Torah requirement, and therefore one may fulfill it with the smaller kezayit. According to this, the one reciting the blessing over the matzah may eat a small kezayit from the upper matzah and a small kezayit from the middle one, and in total he will have the amount of the larger kezayit in order to fulfill the Torah obligation. 

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