I just created another camera and render node for the purpose of taking a picture of the vacinity from the sky downward. I made the render node 4096 x 4096. So when I selected another preview camera that is connected to a render node that has a 16/9 aspect ratio, the preview is a square as though the camera were connected to the map render node but it is not. I would think that the aspect ratio of the preview image would always be that of the render node (with its Master box checked) that is connected to the currently selected camera.
Try Resetting the 3D preview (Reset button upper-left).
- Oshyan