It shouldn't stop you from making larger posters, though. I frequently do. Make the PPT page size half the size of your finished poster, then print it at 200%. Example: To print a banner / poster that is 100 inches wide x 40 inches tall, make the PPT page size 50" w x 20" h. read more
If you change the slide size or orientation after you add content, the content may be rescaled. For PowerPoint 2007 or PowerPoint 2010: On the Design tab, in the Page Setup group, click Page Setup. read more