It seems like Dorico “thinks” of page sizes in terms of pixels at 72 PPI. For example, Letter size is defined in the scorelibrary.xml file as 612 x 792. The 12.9 iPad Pro is 264 PPI so if I scale 2048 x 2732 to 72 PPI, that should be 559 x 745.
I don’t own an iPad Pro, but here’s a zipped doricolib file that will add those dimensions as an “iPad Pro” page size in Layout Options / Page Setup / Page Size. The file also includes an “iPad (8th gen)” size too as that’s what I have. If any one wants to add the doricolib file to their DefaultLibraryAdditions folder, set up a file, then open the resultant PDF in an iPad Pro, I’d be curious to hear if it is defined correctly. The “iPad (8th gen)” size opens correctly in ForScore on my 8th gen iPad anyway.
iPadPageSize.zip (411 Bytes)
EDIT: If added correctly, the 2 iPad sizes should appear near the bottom of the dropdown as below.
Obviously you can edit the file to create other iPad sizes as well.