The posters at ForestSAT2014 have to be vertically oriented in a standard A0 size (1189 mm x 841 mm or 46,77 inches x 33,11 inches).
Here you can download a template (modified from http://www.postersession.com/poster-templates.php).
When you print your poster please take care of using paper obtained from sustainable forest management! For example with a certification from FSC or PEFC.