Is there a standard paper size that is 18"x12"? I’m not aware of one, unfortunately.
When you export a PDF using the ‘Normal’ job type, the PDF will use the page size as defined in Layout Options. But when you export using e.g. ‘Booklet’, then it’ll use the paper size chosen for printing, and I guess the closest size you can choose for printing would be Tabloid, which is 11" x 17".