The Kiddush Cups Of The Chofetz Chaim And The Vilna Gaon
Moshe David Tokayer commenting on Halachic Measurements Chart:
Ok. Here's the skoop on the Chofetz Chaim's kiddush cup:
1. I heard that the cup is in the hands of Rav Hillel Zaks, a grandson of the Chofetz Chaim. He is a Rav in a section of Kiryat Sefer called Brachfeld.
2. Rav Karp in a Hilchos Chag BeChag by Rav Karp in the volume on Pesach, Chapt. 19 footnote 11 writes that the Chofetz Chaim's grandson said that the cup holds 100 grams.
100 grams is the shiur of 2 medium size eggs in their shells. The Chafetz Chaim in O.C. 271, seif katan 68 and the Biur Halacha there says that this is the shiur for Derabanans and Kos shel Bracha. For D'Oraisas like Friday night Kiddush, it is proper to be machmir like the shita of the Tzalach (size of eggs are half now what they were at the time of the Gemara) and use a shiur of 3 eggs in their shells which is a cup that holds 150 grams.
Another interesting item that Rav Karp brings is that the Einayim LeMishpat, a descendent of the Vilna Gaon had the Gaon's kiddush cup and said that it held 120 grams.
100 grams = 3.4 fluid ounces
120 grams = 4 fluid ounces
150 grams = 5 fluid ounces
According to Rabbi Avraham No'eh, the shiur for kiddush cup should be 3 fluid ounces and according to the Chazon Ish it should be 5 fluid ounces.