Page size is are section property. Word defines sections as separated by its section breaks, of which there are several kinds. On your Page Layout tab you'll find Breaks. You need two Next Page section breaks: one before the larger page and one at its end.

Turn on display of hidden characters, position your cursor at the end of the page preceding the larger page, and insert a Next Page section break. If you already have the large page material in the document, insert another break after it. Otherwise, create 2 empty paragraphs after the first break, then insert another break at the second paragraph. This will give you a blank page with the single empty paragraph.

With your cursor at that paragraph, open the Page Setup dialogue and change the page size. Adjust header and footer alignments if necessary for the wider page.