iphone – Set the default paper size for scanning with the pass-through camera

How can I set the default paper size when scanning with a through-camera with my iPhone?

I use the fantastic new Mojave feature to scan documents directly into the preview with my iPhone. My paper is a white A4 sheet. The iPhone automatically detects the edges against a darker background and I take a snapshot. However, when previewed, the image is slightly stretched and the dimensions (cmd + I) show that it has a letter size of 8.5 x 11.

How can I make sure that it is A4?