Quote - I changed the DOCUMENT WINDOW SIZE drom the default 800x800 to 3000X3000 and that did the trick.

Unless you have the world's biggest monitor, your document window must now be off the edge of the screen. Here's an easier way: go to Render > Render Dimensions... (or Ctrl-Alt-D) where you can set the render size independently of the preview size.

My advice would be to keep the render's aspect ratio the same as the preview, but it isn't compulsory.