If you create a custom page size, Dorico displays your page in Print mode on a paper size that you select. So you can put the page on a bigger paper size, or even having the document larger than the paper size!
However, this display is maintained in Graphics export, even though the exported graphic does not get “put onto” a paper size, but is just saved at the document page size.
For example: I create a document of size 210 x 210mm. Print view displays it on an A4 page, or other page size I can select. When I switch to “Graphics”, I can no longer select the paper size, (but I can select orientation), but the print view is still on an A4 page. However, when I export my graphics, I get a 210 x 210mm file. (Which is what I wanted.)
TL;DR: Print View should display documents on their page size, not paper size, when Graphics export is selected.