By default the paper size in MS Office Word is Legal.
But it would be convenient for you change it to A4 permanently, if you are working frequently with A4 size papers (for printouts). You may not have to change it every time you work with a document to print. It will also help you to avoid accidental printouts taken with an improper page size (a very frequent occurrence for me 
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For Microsoft Office Word 2003
- On the File menu, click Page Setup, and then click the Paper tab.
For Microsoft Office Word 2007 / 2010
- On Page Layout tab, select More Paper Sizes option under Size to get the following dialogue, click on paper tab
Set your required paper size (A4 in this case) in there.
Then don’t forget to click on Default button on the left bottom corner, you will get a, as shown above. Select Yes. And you are done.
From now on, you’ll get default page size as A4, when you open MS Office word.