Is there a way to enter a custom paper size in for a Plot?

I recently went through an (excellent) tutorial, but when it came time to layout a plot for export/printing, he referred to clicking on Paper Setup and setting a size of 24"x24". I can't seem to do this. The button says Page Setup, not Paper, and the only size options seem to be those offered by the printers I have installed. When I click "Export to PDF", it seems to take on the aspect ratio of whatever printer paper size I had last selected. This all makes sense in a way, but I could see situations where I might want to specify a final target size for my plot, even though I don't have a printer installed that prints that (for instance I could set my Page Layout size to 24"x24", then export it as a hi res .pdf, then email that to someone who has a large format printer that can handle that size).

Is there a way to do this? Perhaps by installing a sort of proxy printer on my system that allows me to select any paper size I want?