How do I alter margins and Page size in Libre Office?
On Writer page formatting (all of it) is managed through page styles. On the status toolbar (lower part of Writer’s window), between the word count and the language name for the text you can see the name of the page style currently in use: click on that name and, on the Page tab, change the values according to your needs.
You can later create a template from that document and set that template as the default one.