Why the first screen/file is like the picture?
What can I do?

Thank you,
Basile

Please check the Page Setup dialog in the File menu.
You can change paper size there.

Does it help?

Hello Andrey, and thank you for the reply.

When the program starts for the first time, it uses "Custom".
When I changed to "Letter" and save as a default, then the usual square background appears, and it is saved in the next time.

Thank you,
Basile