How Do I Make A Word Document Postcard Size?

The simplest way is to go to File > Page Setup... > Margins (tab) and set:
Top = .75, Bottom = 6.0, Left = .5, Right = 4.25 ; select Portrait as your orientation, then click OK.

Create your ad within the page, print it out. Rotate the printed page and print it again. Flip the paper over and do the same. Now you have 4 ad copies. Cut them out and paste them on another blank sheet and use that as the orignial for the copier and make as many copies as you need to be cut into 4.

The above was just a quick example. You may have to adjust the margin figures to suit your printer requirements. If you want to have a Landscape orientation, then of course you will have to make the proper adjustments as well. There are better ways to achieve what you want, but this is one of the "quick-n-dirty" way of doing a one-off kind of job, and it is easier to understand.  Good luck.

It depends on which version of word you are working in as the 2007 word is different than prior version. Basically it is done through set-up or view and the page Lay-out. You can also do this when you create a new document. You would select the new document when you start word then choose what lay-out you desire at this point.

Find out first the size of the postcard that you want. Then resize the page in MS Word according to the dimensions of the postcard. Change the orientation into landscape so it's easier for you to design and you can have an idea of how it would turn out after printing. Hope that was helpful. :)

Online Printing Company | UPrinting.com