I was just further evaluating the situation with new designs and their window size, and I'm still struggling to understand your point:
In my environment a new tab is always preferred when opening a new document (there is a corresponding system setting under "Desktop & Dock"). A new tab is (of course) having the same size as an already opened window. If no window exists the default window size is used, which is almost covering the whole desktop on my 14" MacBook Pro.
If this system setting is set to "Never" or "Full Screen", a new design is always opened in a new window, again using the mentioned default size.
Could you elaborate your suggestion? Maybe you are referring to the zoom level of an empty document, which is - due to the lack of any design to fit - at minimum presenting a 40 x 40 mm square (approx. 1.5" x 1.5")…?