How can I set the default paper size when scanning with continuity camera using my iPhone?

I am using the fantastic new Mojave feature to scan documents directly into Preview with my iPhone using File > Import from iPhone > Scan Documents. My paper is a sheet of white A4 paper. The iPhone correctly detects its margins automatically on a darker background and I snap the photo. But when it arrives in Preview I find the image is slightly reduced in size in order to fit on a pdf of 8.5 x 11 letter size. I've tried also with other sizes and everything is put on a white letter size PDF background.

How can I make sure that it ends up being the original size?

I tried to change the "Default Paper Size" in System Preferences > Printers/Scanners but to no avail.

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