Question:
Can a person use a DIRABANAN (Rabbinic) shiur (measurement) for the Shabbat morning kiddush LCHATCHILLA? Instead of 4.7 ounce, use 3.5ounce?
Answer:
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.
Best Wishes
Sources:
Biur Halacha 271-13 D:H Shel Reviis, Shemiras Shabbos Khilchoso 47-9.