A family of Microsoft word processing software products for creating web, email, and print documents.
I assume you are talking about the margin width rather than the paper size. You should be able to set margins for the entire document by selecting "Whole document" on the Margins tab of Page Setup. Occasionally, if a document contains section breaks and especially if some of the sections have multiple columns (and apparently always if it is "Book fold" and has more than one section), you won't be able to select this without deleting the section breaks (or temporarily disabling "Book fold"), but you can certainly set the same margins for each section individually.
If the problem is switching from portrait to landscape, then the issue may be "column" width rather than page width. Sometimes text doesn't reflow to a new page width because the column width is set to a specific amount. To correct this, open the Columns dialog and click on the preset picture for One (even if the number of columns is already set to 1). This will check the "Equal column width" box, allowing the text to reflow to match the margin width.