At this time only the predefined page sizes. We will add "any" page size shortly.
Suggestion: You can use the export PDF feature (menu: File / Export / PDF). It's slightly different from Print to PDF because it creates a PDF that contains vector objects (circles, lines, etc). It uses the extents of your design to determine page size, so you can easily add fiducials (or markings) which signify the actual page size you require. I'll add a tutorial to the FAB 3000 forum shortly showing how to export PDF files (to any page size).