I have split my document up into 11 chapters, but forgot to adjust the length of the header before splitting the document, and now I have to manually change each chapter heading section individually. Is there a way to select them all and do it once?

Unless the document contained large amounts of graphical objects, there was probably little or no need to split it - Word can handle file sizes of up to 512Mb. Other than that, manually resizing the headers in the individual documents would take much less effort than trying to automate it.

__________________
Cheers,
Paul Edstein
[Fmr MS MVP - Word]

It's a book. I split it into chapters with headings with section breaks.

The same can be done in a single document, too.

__________________
Cheers,
Paul Edstein
[Fmr MS MVP - Word]

To clarify:
What is the difference between headings and headers?

I take it that you have a single document with 11 chapters. A section break is not needed for that.
If you used the built-in Heading styles for your chapter titles, you can simply modify the heading style to allow additional space before and a page break before. You may need to put your heading 1 style in a Frame to achieve the Space before. See screenshot below.

If you want to change the headers to reflect the content of the chapters, that is the function of the StyleRef Field.

Understanding Styles in Microsoft Word
Sections / Headers and Footers in Microsoft Word 2007-2016

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