Using Ctrl+n we are effectively doing FrontEndTokenExecute["New"] and, at least on Windows, a new window has a WindowSize equal to the current Input/Selected Notebook's WindowSize.

Can Ctrl+n produce a notebook with fixed/predefined size? Is there any option for that?

This method works for things like CreateDocument etc. but not for my case.

There is also something like CurrentValue["DefaultWindowSize"] but it doesn't seem to affect Ctrl+n.