Postcards are normally treated as labels with three or four cards to a
sheet. For individual cards you can create a separate document template,
which you could base on an envelope template - see
http://www.gmayor.com/changing_envelope_layout.htm  - or you could build it
from scratch and place the address information where you want. Note that
Word will only create document sizes defined in the printer driver. For non
standard sizes, you need to use a large standard sized 'paper' setting and
make up the difference with margins.