Libreoffice invokes the OS printer settings dialog in such a way as prevents the user from being able to set the paper size. When invoking the the OS printer through File->Printer Settings, the paper size is selectable. However, when invoking it through File->Print, the paper size is fixed to what LibreOffice thinks is the closest paper size. This leads to certain printer features being disabled, since some printer drivers have multiple paper size settings that are the same physical size, but different in that they invoke different printer features. Preventing the user from manually setting the printer page size prevents those settings from being accessible. It also seems to be a case of the software telling the user what the user should be doing.
Created attachment 150643 [details] Screenshot of paper size selection in LibreOffice 6.2
With LibreOffice 6.2: After selecting the printer in the print dialog, please click "Properties" and then enable the "Use only paper size from printer preferences" checkbox. After that, you will be able to select he paper size from the dropdown menu (s. attached screenshot). In current master (to become LibreOffice 6.3), the print dialog has been reworked and thus looks a little different, but paper size selection is possible there as well. I'm setting this bug to WORKSFORME. Please reopen if you're still unable to set the paper size using the described steps and give more details.
*** Bug 124636 has been marked as a duplicate of this bug. ***
Thank-you very kindly for your patience in responding to this. It was indeed a PEBCAK. I apologize for the spurious report. This also resolved 124639.
No problem, great to hear it's working now.