Add option to choose page size (A5, A4, etc.)

[jasle]

It would be very useful to have the ability to select different page sizes, such as A5, A4, and others. Currently, the page size appears to be fixed, which limits flexibility for users who prefer specific formats for note-taking, printing, or exporting.

Suggested improvement:

• Add a setting that allows choosing from common page sizes (e.g., A5, A4, Letter, etc.)
• Optionally allow custom page dimensions for even more flexibility

This would make Notable better suited for users with different workflows, printing needs, or personal preferences.

[Ethran]

Again, as with backgrounds: it will require a lot of work, and not much benefits. It should be addressed in future, when app is more mature.

Btw, thanks for your opinions!