How do I export ink separations at artboard size?

Is there a way to export out ink separations at artboard size? I am trying to export ink separations through Ilustrator. Using the instructions from Adobe [https://helpx.adobe.com/illustrator/using/printing-color-separations.html], I am able to create a .pd through the print dialog. The only issue is that even though we are setting the printer to Postscript, I still seem to be limited to the size limitations of the printer listed on the PPD. Setting the PPD to Generic Postscript eliminates the Custom Size option, and selecting Device Independent completely eliminates the option to create the separations. 

Any help on this or alternate methods? Mainly just need to export the white ink plate at size.