PC's use a base resolution of 96 ppi. Mac's use 72 ppi.
A traditional PPT slide was 10" x 7.5" on the PC, totaling 960x720 pixels.
Now, PowerPoint will allow for larger resolutions for your images. However, when displaying, your images and fonts are rendered at the screen resolution -- which could be set by your monitor or a presentation switcher.
It is all very confusing.
So, if your goal is HD -- then 1920x1080 is great. If you don't know, a resolution of 150 ppi works great for both onscreen and desktop printing.
BTW, PNG images have great advantages in ppt. You can use Photoshop or illustrator to export them. They compress in PPT and print great, and allow for transparent backgrounds. If you are old-school, and have built massive decks, you know JPEGs can cause all sorts of problems.
Oh. I have told you too much ...