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.
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