OK, I increased the line size and was able to increase the Exported MFD up to 512x512. I also had to fix the fonts which were too small to be legible.
You'll notice the exported MFD is taller than the one in the cockpit, so it shows more map area than the cockpit screen does.
Your position indicator on the exported MFD is a green circle rather than the big cross hairs. I can change that if anybody cares.