What’s the smallest Shiur in ounces for the four cups of the Pesach Seder?
I understand that the 1st is דאורייתא so its probably more. But for the first and the last 3.
The following are the minimal amounts one must do on Pesach:
The cup(s) must hold a revi’is of wine, or diluted wine or grape juice if necessary.
The size of a revi’is according to R. Chaim Naeh is 86 grams which is 3 ounces.
One must drink at least enough wine to fill one side of the cheek; this is a majority of one revi’is. In other words, one must drink at least a majority of 3 ounces for each of the first 3 cups.
For the last cup in order to recite a after blessing, one should drink an entire revi’is. Alternatively, one may be yotzei the bracha acharona from someone else who drank a full revi’is.
In pressing situations, one may use chamar medina for the arba kosos. Examples of chamar medina are coffee or tea.
See Shulchan Aruch Admur Hazaken 472:17-21; 27-29. 483:4. See also here and the references in Hebrew
PS. According to halacha, Kiddush on Yom Tov isn’t deoiraisa and even on Shabbos if one davened Maariv already.