Missing custom paper size in the printer properties menu

Hi all,
I have an Ubuntu 1504 box with HP Photosmart 6520 connected to it.
I successfully managed to declare a custom paper size in the cups printer driver and indeed the new page size is present in the list of HPLIP Device Manager >> Print Settins >> General >> Media Size.

But this new format is not present in the dropdown list I find in Print>>Properties>>Paper Size within LO.

How can I align the two lists of available media?

Thanks in advance,

I think the best way to handle this is to use a template within LibreOffice. You can find a good bit of information in the LibreOffice user guides. Create a new, blank document. In Format - Page, modify the page size to your new paper size. The page size type shown will change to ‘User’. Save the blank document as a template. Then when you want to use that specific paper size, open the template from File - New - Templates, type your document, save it, then print it after selecting the correct paper tray. The paper sizes LibreOffice uses are stored within LibreOffice and are not dependent on the settings in the CUPS driver as far as I know.