Evidently, you have done something to change the page size in Word's Normal template. Either:
1. Open Word's Normal template and re-set the page size; or,
2. If you have no other customizations/macros in Word's Normal template, exit Word, delete the template then re-start Word.

Word templates are normally stored in:
C:\Users\%Username%\AppData\Roaming\Microsoft\Temp lates\
File names end in .dot, .dotx or .dotm

To open a template for editing, select the 'All Word templates' option in Word's File|Open dialogue.

This won't affect any existing documents and it's not apparent how such a 'corruption' affecting a template could ripple through to your existing documents.

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