I have a situation with the following constrains:

.docx only, no macros
section breaks must use the "same as previous" header/footer

Now, I have a table in my header, page size Letter, portrait orientation. Sometimes we need to insert a section break and change the page size/orientation, i.e. 11x17 (tabloid) with landscape orientation.

Since the header/footer must be same as previous, you cannot change its size as it will affect the previous pages on Letter size.

is there any way to auto size the width of the table in the header so it will always be the full width of the page?

You can't do it with a table, but you can with alignment tabs, if that will meet your need. These tabs align left, center, and right in relationship to the page margins, not absolute position.

Sections / Headers and Footers in Microsoft Word 2007-2013