How do I choose default paper size for printing?

The current default size is “Letter”, I want to change it to “A4”.

Unless you modified your default template Default Page Style page style, Writer retrieves the default paper size from the printer driver.

Fix the default paper size in the OS printer configuration.

Since you didn’t mention your OS name (and also LO version), I can’t give a more precise procedure.

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Thanks for your answer, which is correct but not detailed enough for a informatic infant like me. For my fellow babes, I found this :

In that case you’re in trouble, because paper size selection is made by each printer’s driver. It’s not a Windows 10 setting.

Go to Settings > Devices > Printers and Scanners > click on your printer and click on the Manage button.

When the new windows opens, click on Printing Preferences. That’s where you select your default paper size.

Then I deduce you’re under Windows 10. Every system and even every version of a system has its own way of configuring default settings. Beginners in computer lore should always describe precisely their configuration.