There's no particular size in pixels.  PowerPoint scales all content to fit the
display or printer page it's displaying on at the moment.

If you intend to create templates that'll be used for on-screen presentations,
start with a default-size PPT presentation (page size set to On-Screen Show)
and assume that a full screen image will be 1024x768 (the size for most data
projectors).

You'd save it as a Design Template (.POT) rather than as a normal presentation,
assuming you're using PPT 2003 or earlier.

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Steve Rindsberg, PPT MVP
PPT FAQ:  www.pptfaq.com
PPTools:  www.pptools.com
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