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
This is brought by Ha-Rav Moshe Karp as testified by the Chafetz Chaim's grandson, Ha-Rav Hillel Zacks, the Rav of the Brachfeld neighborhood in Kiryat Sefer, who inherited the Chafetz Chaim's Kiddush cup. Hilchot Chag Be-chag – Pesach, Chapter 19 note #11 (and also brought in Meged Givot Olam Vol. 2, p. 34).
Another reference to the same source says
“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.”
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.)
In a recent lecture (starting at about 1 hour and 46 minutes in) R. Aaron Rakeffet discussed a story, which he claims is "absolutely true" and he remembers it and "all of Jerusalem was shaking". Apparently, after R. Mendel Zaks died his widow (who was the Chofetz Chaim's daughter) would have a yeshiva student come to her home to make kiddush for her. One student told her that the cup was too small, to which she replied that this was her father's (the Chofetz Chaim) kiddush cup.
Rav Dovid Cohen of Chevron told over (Summer Zman 2017) that Rav Hillel Zacks indeed had the Chofetz Chaim's Kiddush cup with the smaller shiur. Rav Hillel however did not use it because the klal "is noheg to be machmir in these areas".