Hello Steffi,
from many handed-over BV I can tell you that double-winged, floor-to-ceiling windows in sizes < 1.51 m don’t make much sense if they are to be used practically. Furthermore, it is so that humans are creatures of habit and therefore the routines from the rental apartment are also reflected in the owner-occupied home. You will surely also transfer your "paths/actions" from the rental apartment to the new building. Example: if you currently open all windows with your right hand, you will handle it the same way in the new building ... if you currently live on the ground floor and always go through the left window door onto the terrace, you will do the same in the new building. This must always be taken into account when arranging/ sizing the windows.
If your picture from #5 corresponds to the later picture of the new building, I would go for 0.885 x 2.135 m (1 window sash) on the right and left and as wide as possible in the middle, but at least 1.51 x 2.135 m. This ensures that you can "sometimes" walk outside onto the terrace with a tray through one of the outer window doors without having to squeeze through. In the middle, you can then - fully opened and without mullions - also transport bulky things.
The illusion of a "divided" window door can be maintained by internal mullions; if it is about the optics for you.
Rhenish greetings