How to use templates in other page/paper sizes
By default, included templates are US Letter size. But what you can do is select the new page/paper size under Document > Page Layout, then File > Templates > Save as Template as something like “Template Name (A4)” (or whatever you’d like, really).
You can also choose the option to automatically install the newly saved template when saving.
Then you can use select that template from File > Templates > New from Template, or even set it as the default startup document under Preferences > General.