How do I choose default paper size for printing ?

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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.