Slide size - pixels and inches in Keynote '09

The important thing is to get the ratio right (42/36 = 1.1666...). Then you can scale it when you print. None of Apple's default sizes (800x600, 1024x768, etc.) have the ratio you want, but you can use a custom size. The following discussion describes how to do that, and generally is the same question as you have (and also has the same answer -- even if you get the aspect ratio right, Pages is better).

http://discussions.apple.com/thread.jspa?threadID=1481437

Message was edited by: pbg

Keynote is designed for on-screen presentations, so it measures in pixels. You would be better off doing your poster in Pages, which is designed to produce printed media (and thus measures in inches).