I read somewhere (don't remember where) that the Chofetz Chaim used a kiddush cup that was a smaller shiur than he recommends to use in his Mishna Berurah. Can anyone verify this and/or explain the details?
I have heard the same rumour. I found the following by Rav Aviner. He was asked that whether the Kiddush cup of the Chafetz Chaim was like the measurement of Ha-Rav Chaim Naeh and not like that of the Chazon Ish.
His answer was
Another reference to the same source says
Shmiras Shabbos Kehilchoso vol 2, 47 (9), gives the shiur as 1.5 eggs with the shell which is according to the minhag Yerushalayim 86 mls which he says is the custom of the Edos Hamizrach. He quotes the simple minhag outside of Israel as being 137 mls (based on the Kitzur Shuchan Oruch, Feldman and the Mishna Berurah O Ch 271 (13) MB ) and the shiur of the Chazon Ish as being 150 mls.
So it seems that the Chofetz Chaim's cup of 100 grams is less than the 137 mls ascribed to him (the MB does not give shiurim in mls.) (I assume a specific gravity of wine of 1.)