Since Powerpoint is mostly used for screen presentations, going with screen resolutions would be a good answer.
The most common are:
640x480 (same as television)
800x600 (very popular)
1024x768 (my favorite with XP, OS/X and Linux)
1280x1024 (the new large high res screens)
Depending on what hardware you plan on using for display and if you plan on doing zooms and stuff choosing the correct resolution will give you the best quality/file size ratio.
It is useless to use greater resolution than what you plan on displaying as you will only waste space.