Insert Separate paper sizes on the same doc – Microsoft Word.
Set multiple page size in a single Word document following some easy steps. You can mix several page sizes forever not for temporary session. Following this way, you are able to add specific paper sizes like Letter, Tabloid, Legal, Statement, Executive, A3, A4, A5 etc.

You have to insert section breaks between the paper size changes:
Page Layout tab Page Setup group Breaks drop down: you would want to use a next page or if you are duplexing, ODD Page break.

If you are just creating the doc, then simply enter section breaks as you go. If you are working in an existing document, insert sections breaks at start and end of pages that need particular page sizes BEFORE changing the page size.

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