Unfortunately, the link above only tells you how to set new windows to open at a specific size. But it doesn't even hold across restarts and/or Finder quit.
In order to do what you want, you have to have a 3rd-party utility like
FinderFix or similar. FinderFix not only allows you to set the specific size of every window when opened, it allows you to tell it where to appear on the screen (like 200 pixels from the top and 500 pixels from the left, for instance).