If you have a paid account and see the Large (300%) and Extra-Large (400%) screen size options grayed on the Export screen, and you are also using the Chrome browser, this is due to the diagram being too large for the browser stack to support exporting at that size. This is a limitation of Chrome's memory capabilities.
Please try using the FIREFOX browser instead. If using Firefox does not resolve your issue, please email firstname.lastname@example.org.
You still must have a paid Gliffy subscription to export as an image.