Can a person use a DIRABANAN (Rabbinic) shiur (measurement) for the Shabbat morning kiddush LCHATCHILLA? Instead of 4.7 ounce, use 3.5ounce?
Yes, Kiddush on Shabbos morning is only D’rabonon, (unless he person didn’t make Kiddush etc. on Shabbos eve). Therefore one may use a smaller measurement. As a side point the shiur according to R’ C. Naeh (the smaller shiur) is 2.93 fl. oz., and according to R’ Moshe it is 3.31 fl. oz.
As a side point, I personally saw one of the gedolei yisroel who regularly used a larger cup for Shabbos eve, and a smaller one for the Shabbos day Kiddush.
Biur Halacha 271-13 D:H Shel Reviis, Shemiras Shabbos Khilchoso 47-9.