Longer posters will work, they will just come out as more than one page. Use Adobe Acrobat to produce a pdf version of your poster.
Distiller does a sophisticated job of compressing images.
It is usually necessary to use Distiller if you include 3D fonts form Power Point. Check with DiTRL for more detailed instructions. Adobe Acrobat is installed on several of the DiTRL machines. Have a look at an example pdf poster. Digital Teaching Resources Laboratory - Department of Biological Sciences.