Select that paper size in the File>Page Setup dialog, or just create the
document as A4 and then when you print, select 2 pages per sheet in the Zoom
section of the File Print dialog, or see the article "Booklet printing" at:
http://www.word.mvps.org/FAQs/Formatting/BookletPrinting.htm
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Hope this helps.
Please reply to the newsgroup unless you wish to avail yourself of my
services on a paid consulting basis.
Doug Robbins - Word MVP