First, According to those that are saying that kiddush you shod be Machmir with the shiur, then you should be machmir every Friday night.
Second, in Shulchan aruch it only mentions a machlokes regarding the kzayis for matzah, and paskens that for doyraysa (Motzi matzah) use the bigger shiur, and for drabanan (korach) you can use the smaller shiur kzayis. But nowhere does it mention to be machmir by a kos,if it is a doyraysa kos, to use the bigger shiur. If your are machmir to use the biggest shiur by mitzvos in general, then be machmir by the first kis as well. If your not usually machmir in general for the bigger shiur in mitzvos than you won’t have to be machmir here. If your going to want to use the Chazon Ish’s shita for the first kos, you might as well wear the long tzitzis all year round.
