Do German Ashkenazim sit or stand for kiddush and is the custom the same in the evening and day and for Shabbas and yontif?
They sit during all kiddushim. Source: Personal experience from countless families, including Rav Shlomo Breslauer of Beis Tefilla, Monsey, NY. Beis Tefilla is the large Jeckische (German Ashkenazi) synagogue there (other than the Frankfurter synagogue, KAJ).
According to the Yekkish site KAYJ:
... another minhag that was in the past and today is more common, is to stand both at night and in the day, throughout the whole Kiddush
(It also mentions the same for Havdalah)