When going to the “Print” section, on the top right under “Destination” select “Printer”, and then below under “Page Setup” select any format, e.g., “89x89mm”. Then go back to the top, and under “Destination” now select “Graphics” instead. As you see, now under “Page Setup” below, that format shows up as well, but grayed out.
Now my point is this: Clearly, when exporting as a graphic, the concept of page size (by which the system means “the page you print on”) is completely irrelevant and shouldn’t even show up there, not even grayed out. When exporting as a graphic, all that matters is the page size chosen under Layout Options.
I know you may now say, “this is why it is grayed out”. But given that things can get quite confusing when dealing with slight variations in page sizes and how scaling (“fit to page” or custom percentage) can have unintended effects, I think this text field should rather be eliminated entirely when “Graphics” is selected.