If you have Adobe Acrobat, it installs a printer driver that makes PDFs when you
print to it (and that allows you to set your preferred page size).
There are several other products, some free, that also install printer drivers that
work similarly. CutePDF, PrimoPDF are a couple of examples.
PPT's own Save As PDF add-in doesn't seem to respect your chosen page size in PPT,
so it may not be possible to force the page size to anything other than its default
size.
Out of curiosity, does it present a problem when the PDF isn't 8.5x11?
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PPT Frequently Asked Questions
http://www.pptfaq.com/
PPTools add-ins for PowerPoint
http://www.pptools.com/