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 (Pesaḥim 108b).
A revi’it is equal in volume to an egg and a half. According to the calculation of Rambam and other Rishonim, this works out to c. 75 milliliters (Peninei Halakha: Berakhot 10:11; according to R. Ḥayim Naeh, the quantity is 86 ml). This quantity, however, is not agreed upon by everyone. The exiles gave rise to uncertainties regarding the size of eggs, and some later Ashkenazic poskim (Noda Bi-Yehuda, Ḥazon Ish) stringently rule that today’s eggs are only about half the size of eggs in the time of the Sages. If a revi’it was the same volume as 1.5 eggs, it is now almost three eggs, or c. 150 ml. This stringent measure is known today as a “Ḥazon 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 aḥarona, one may fulfill the mitzva using the smaller measure, in keeping with the view of the vast majority of poskim. Nevertheless, le-khatḥila it is not proper to fulfill the mitzva using the smaller amount. Rather, one should fill a nice goblet that contains much more.
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. The cup, however large, must be filled; even if the cup contains much more than the amount with which one fulfills his obligation (a revi’it, or 75 ml), it must be filled, so that one does not appear miserly. This does not mean that the cup must be filled to the brim, to the point that there is concern that the wine will spill. Rather, it should be filled generously, almost to the brim (c. half a centimeter below the brim).
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-ḥayim 473:1).