how do I increase the size of a slide so that I can print the slide as an academic poster? (45 x 37 inches, high res print)

I'm using keynote in iwork '09 suite.

I have tried exporting the document as a pdf, or image file (jpeg or tiff) to re-size in another application. However, exporting in these formats produces a resolution too low for printing (72 dpi).

Has anybody figured out how to make posters from keynote?

Thanks for your suggestions!