Scientific poster with Keynote - 91"x45"?

I have made scientific posters in Keynote. It is the best tool I have used for this task, starting with the templates provided by Apple. The current poster I need to make is much larger than ones I have made in the past (4000x3000 at 72dpi).

I need to make a poster that is 91"x45". In Document -> Inspector, I can't set slide size in the custom settings beyond 4000x3000. I would need a slide size that is 72 (dpi) * (91, 45) in dimensions, right? In Pages, I can use such sizes, but given the amount of time I have, Pages is not an option for this poster.

Is it possible to use such large dimensions? I would be happy to make a scaled version and have the printer (Kinko's) scale to the proper size. For example, I could make a 45.5 by 22.5" poster and have Kinko's double the size. If I use this strategy, what do I need to worry about? All the included figures are PDFs but some of these PDFs have embedded raster graphics. Will they scale poorly? Do I have to use a certain DPI setting in my small scale poster?

Thanks!