ianicholls,
Go to File - Page Setup - choose one of the sizes or make your one size
using the custom item and set slide orientation to Portrait.
If you are printing the poster, see here:
http://pptfaq.com/FAQ00537.htm

Signature
Luc Sanders
MVP - PowerPoint
blog: www.sixtiesfan.multiply.com/
>I want to create a poster in PowerPoint 2007, but I cannot find how to
> change the page dimensions to 3 ft by 4ft.
>
> Thanks.