Here is an excellent explanation on Microsoft PowerPoint resolution.
PowerPoint defaults to a resolution of 720 x 540 (approximately the same as digital video). In PowerPoint, select File>Page Setup and you'll see the page is set to 10 x 7.5 inches (25.4 x 19.05 cm). The problem with this dimension is that PowerPoint will scale your presentation visuals either up or down, depending on your current display setting (see above).
So how do you fix this? Using 1024 x 768 for this example, divide each dimension by 72 ppi (pixels per inch). 1024 divided by 72 = 14.22 inches (36.19 cm). Enter this for page width. 768 divided by 72 = 10.66 inches (27.08 cm). Enter this for page height. Your PowerPoint file is now perfectly set for a 1024 x 768 display.
Here is how to find Page Setup under PowerPoint 2010.