Units of the size in open XML is EMU (English Metric Units). And you can convert those to pixels with following formulas.
pixel=EMU * Resolution/914400;
where "Resolution" is resolution of your screen. e.g. 96 dpi etc.
For more details go through below link.
In general, use these formulas:
These formulas work well with the default numbers used in the PowerPoint client - especially images which are usually sized for 72 dpi, regardless of their internal dpi or the dpi of your screen. In other words, the way a PowerPoint creates a slide is for printing first, secondarily for on-monitor-display so print ppi/dpi does actually matter.
So in the case of your question above, if you want your slide to be 10" wide (SlideSize.Cx), you would set it to 9144000 (=10 * 914400) using #3 above.