Can't change default paper size in word 2010.

If the old format shows up again when you restart Word, then clearly your changes weren't stored in the default template (normal.dotm I'm assuming). How did you change the format?

Open Word through the start menu (a blank document opens). Go to "Page Layout" tab => "Size" button => "More Paper Sizes...". Choose the size you want and click the "Set as Default" button at the bottom. If it asks you to change the normal template, click yes. Close Word. You don't have to save the empty document. Now restart Word again. Go to the "Page Layout" tab and check the "Size" button. Your selected size should now be applied to the document.

Just for clarity, opening existing documents will not update their page size to the newly selected page size. They will keep using their original size!

If it still doesn’t change the page size, then the template might be corrupt .You need to reset word back to its initial settings and change the page size again. Follow method 1, 2 and 3 in the article below to reset word:-

http://support.microsoft.com/kb/921541