Dear Rabbi Simon
I want to get a new kiddush cup for use on Shabbat/Yom Tov, Havdalah, Seder night, etc. I found one which holds 90ml/3.04oz. Is that large enough?
Thank you for your practical question.
As a matter of basic halakhah, you may use a cup of the size you mention, as you can rely on the widely-accepted view of Rav Hayyim Naeh (d. Jerusalem 1956) that the minimum volume of wine (or in some cases another beverage) for kiddush, seder night, etc., is 86 ml. However, since this will necessitate always filling the cup nearly to the top, and also in deference to the view of the eminent authority Rav Moshe Feinstein (d. New York 1986), I would advise choosing one which holds 100 ml+.
Just for the sake of completeness we should also mention the view of Hazon Ish (nom de plume of Rav Avraham Yishaya Karelitz, d. Benei Berak 1953), that a kiddush cup should hold not less that 150ml. There are those who adhere to this opinion as well (although I would not regard it as necessary to do so).
Best wishes and L’Chaim
Rabbi Rashi Simon