It took a long time to figure out that if you select File->Scan and enter a page size under the Geometry tab and then cancel and go to Edit->Preferences and click Apply, you can get the page size to default to whatever you selected when opening the application.
1 - I'd like to see the default page size under Edit->Preferences.
2 - I don't believe that applying the preferences should save the page size selected under File->Scan. The new default page size should only be able to be set under Edit->Preferences. It should then be the default for the File->Scan interface. (They should be mutually exclusive except for when the File->Scan page size is initially set to the default page size on startup.)