In order to fulfill the mitzva of the four cups, or any other mitzva that involves drinking wine (such as kiddush, havdala, Birkat Ha-mazon, and wedding ceremonies), there must be a significant amount of wine in the cup. The Sages determined that the cup must contain at least a quarter of a log (a “revi’it”) of wine. Less than this is not a significant amount of wine and does not suffice to fulfill the obligation (Pesaĥim 108b).
A revi’it is equal in volume to an egg and a half. R. Ĥayim Naeh calculated, based on writings of Rambam and other Rishonim, that this is 86 milliliters. However, more precise measurements showed that it is c. 75 ml (see Peninei Halakha: Berakhot 10:11). This amount, however, is not agreed upon by everyone. The exile gave rise to uncertainties regarding the size of olives and eggs, and some later Ashkenazic poskim (Noda Bi-Yehuda, Ĥazon Ish) rule that today’s eggs are only about half the size of eggs in the time of the Sages. Therefore, a revi’it is closer to the volume of three of today’s eggs, c. 150 ml. This stringent measure is known today as a “Ĥazon Ish shi’ur.”
In practice, the lenient opinion is the standard, and this is the practice of Sephardim. However, MB 271:68 and 486:1 states regarding the practice of Ashkenazim that it is best to take the stringent opinion into account with regard to mitzvot of Torah origin like kiddush and havdala. However, when it comes to rabbinic mitzvot like the four cups at the Seder or the minimum amount that must be drunk in order to recite a berakha aĥarona (a blessing recited after eating or drinking), we use the smaller measure, in keeping with the majority opinion. Those who wish to be stringent, and who enjoy drinking wine, are commendable.
One must also take care to meet the Sages’ requirements for a kos shel berakha (a cup of wine linked to the performance of a mitzva). A broken cup must not be used, and the cup, however large, must be filled with wine in honor of the mitzva.
The cup must be clean, thoroughly rinsed inside and out, before the first cup. However, as long as it has not become dirtied, it is unnecessary to wash it again for subsequent cups, since all four cups are considered one continuum (MB 473:68). Nonetheless, some take care to wash the cup before drinking each time (Kaf Ha-ĥayim 473:1).
. Kaf Ha-ĥayim 472:1 states, based on Zohar, that one should rinse the cup again before Birkat Ha-mazon. Some had the custom of preparing a basin of water and immersing the cups in it between each drinking. Nowadays, this practice is not as nice, since the water in the bowl becomes dirty; we would not consider such cups clean. One who wants to be strict should rinse his glass in the sink. Most poskim maintain that one need not rinse out the cups in the middle of the Seder.