We have some customization which changed the default size from A4 to "A", plus a lot of other customization. I can't tell you exactly what the process is because we have some much other customization. But I can tell you that we have a file called "am_Plot_PDF-US.lsp" which specifies a printer to use: ":PDF_US_A". And we have a file, "am_Plot_PrinterStyle_PDF_US_A.lsp" which specifies the paper size (as "A", the orientation, and the type as ":PDF". I believe that the files were created by just going into the Print menu, setting the printer type to "PDF", and then adjusting the paper size and other parameters.
I can't tell you where to put those files because your setup is going to be different than ours.