One way is to go into plot and select the printer as dwg to pdf.
open the properties and under "user-defined paper sizes & calibration" there is "custom paper sizes", click on this and then you can add a new size, you can then set up a custom paper size here, once you have gone through this your new size should be selectable in "Paper size".
![Autodesk Design & Make Report](https://forums.autodesk.com/t5/image/serverpage/image-id/1350519iCB55845EBFC43016/image-size/large?v=v2&px=999" "Autodesk Design & Make Report")
