How do I change page size when print to postscript file?

I am using LibreOffice on Mac OS X El Capitan 10.11.6.

My questions is about exporting a document I am working on into a postscript file.

My document is using a custom page size (19cm x 19cm), and all page styles used in the document have been updated to use the exact same page dimension (i.e. using Format → Page… → [Page] tab).

If I choose “print preview”, I can view the file normally — i.e. presented in a square format (19cm x 19cm), all content formatted properly and fitting as designed within the page format.

If I go to Print, in the preview it shows an A4 page, and the content crop-fitted onto the A4 page. There is literally no option to choose the print paper size anywhere in LibreOffice’s print dialog. If I ignore this and proceed to export the file as either PDF or PostScript, what happens is that it is exported erroneously as previewed in this dialog, i.e. an A4 page with “square” content (19cm x 19cm) cropped-to-fit.

I have tried to create a new “Generic Postscript Printer” through the Printers & Scanners system preferences pane, and I have even tried to set the Default Paper Size to “square” (defined as 19cm x 19cm), but that has no effect.

If I choose “Export to PDF…” then it exports it with the correct dimensions, and the page size of the exported PDF is correctly 19cm x 19cm, content formatted properly etc.

Can anyone help me print this document onto a postscript file and change/choose a page size which corresponds to that of the document, so that there are no discrepancies?

Exporting to PDF unfortunately isn’t an option in my case — I want to export this document (approx. 200 pages) into PDF in order to send to a publishing house for printing; however, they require that the PDF file has not been produced using a Mac Quartz method, which is what happens when LibreOffice exports the file as PDF. I am therefore trying to export it as a postscript file in order to convert it into PDF using the company’s own preset for Adobe Distiller, but I can’t get to export it correctly as postscript.

Any help would be much appreciated! (Apologies I can’t upload pictures or files, apparently I don’t have enough “karma” points…)