FROM THE STAR-K GUIDE TO PESACH
This is beyond accuracy, legitimacy, and intelligence. Puleeze! Saychel, apparently, is an eideleh zach.
. Kizayis Measurement2 – 1.27 fl. oz. (38 ml) – If one eats a kizayis of bread, he must recite birchas hamazon. Similarly, if one eats a kizayis of any other food, a bracha acharona must be recited.
Our testing indicates that this is the approximate size of a golf ball (40 ml) or a roll of quarters (37 ml). Approximately seven Manischewitz Tam Tams comprise a kizayis. Therefore, if one consumes this amount, an al hamichya blessing is required. A complete graham cracker (both halves) has a volume of approximately 27 ml. Therefore, if one eats only one graham cracker, a bracha acharona is not recited. An additional one half must be eaten in order to say the bracha acharona. A bracha rishona is required even for any amount less than a kizayis from which one is neheneh (derives benefit).
When eating bread or cake, one should calculate in his mind whether it has a volume of 38 ml (e.g. whether it has the volume of a golf ball). It is impossible to state how many slices of bread equal a kizayis, as bakeries and bread companies bake bread in many different sizes.
On the first two nights of Pesach one should eat two kizaysim of matzah at motzi matzah.5 This is equivalent to approximately one half of a standard size machine matzah or one third of an average hand matzah. If eating this amount is too difficult, one may eat one kizayis – i.e. one quarter of a machine matzah or one sixth of an average size hand matzah. A kizayis is also the amount of marror one must eat to fulfill that mitzvah.6