I do all my parts printing on SRA3 (oki C911dn) in booklet form.
I wondered if there was a place where the paper sizes were defined.
I found a place in scorelibrary.xml
So, I backed up the file, and created some new XML sections for SRA3 and SRA4. I had to convert from mm to points, but it then (when I restarted Dorico) showed up in the layout configuration.
Just wondering if it will work, print preview seems to work ok. I created some new type ids (added sr to the existing .a3 and .a3 IDs)