Hallo, wie kann ich mein papierformat individuell gestalten?

If you want to use a custom paper size that does not appear in the box 3 options, you may need to use the box 5 options and scroll down to the bottom where the Custom option will open another dialog for the size of the photo.  If your photo size then matches the custom paper size it will show up as user defined in box 3.

You may also need to use Page Setup dialog to make changes in the printer driver:

For Photoshop Elements 2024, Editor, open the Advanced tab.

Go to File>print. In the dialog that opens, you can select the paper size under #3, and the orientation as well.

Let us know if this works for you.