Printing an A3 (Word) document on an A4 printer

As you suggested, exporting to a PDF first is another way this can be achieved without the use of specialist software.

This feature requires Adobe Reader X (10) or above.

Open the print dialogue and click the poster printing button:

With my A3 file, this by default put it onto a 2*2 grid (i.e. four bits of paper) which certainly isn't ideal. However by experimenting with the value for tile scale you can adjust this to make it down to 2*1:

This does obviously slightly scale down the size of your image. However, by reducing the size of the margins in your printer settings you can reduce this effect - with borderless printing Adobe Viewer is quite happy to fit the A3 page onto 2 A4 pages with a maximum tile scale of 94%.

If your printer doesn't support 'banner printing', here's a free application that will do the job (just save the document as A3 sized image):

Easy Poster Printer enables you easily to print posters of any size (max 20x20 meters). Just drag n drop your picture into the application and click print.

Another free (and open source) Poster Printer: