You can easily change a document's page size at any time.

Doing so, though, can entail numerous other changes, such as to paragraph indenting, font sizes, and so on, in keeping with the change in format. Where pictures, tables and the like are concerned, you might also need to do a fair bit of resizing, as they don't auto-adjust (tables might, depending on their formatting) and could end up on the wrong pages or in the wrong places on the right pages. Depending on the publisher's requirements, it might be acceptable to submit a PDF. The advantage of this is that PDFs can be re-scaled without any changes to the existing layout.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]