Some time ago, we had a need to for a much larger image file of our layout that was provided by the screenshot feature (File -> Screenshot).
At that time, Matthias was kind enough to provide some instruction for how to do this through the Ruby interface, so I thought I'd record that here in case anyone else finds it of value.
Tools -> RBA Console
In the command line along the bottom of the window, enter the following command, then hit Enter.
REPLACE <xpixels> and <ypixels> with the size you desire. In our example, we once output a 10,000 by 10,000 pixel image file.
After completed, command line will clear.