As Luc says no easy way. IF you are competent to unzip the pptx file and modify the XML it can be done, the slide size will change but the pictures will not change (they will move though and you will have to adjust the positions)

Unzip > look for PPT > presentation XML and change the slide size values (p:sldSz) then rezip.  Don't go there if this makes no sense and in any case keep a copy!

Relevant values are:

4:3         cx="9144000" cy="6858000"
16:10      cx="9144000" cy="5715000"
16:9        cx="9144000" cy="5143500"

This idea originally came from Steve Rindsberg.

Jay RB,

Unfortunately there is no way to do that. You can make a copy of your presentation, change the dimensions, then copy the distorted stuff from the original ppt in your copy. I know, not a pleasant prospect, but maybe someone will have a better solution.

Luc Sanders
MVP - PPT