The default settings are 33.867 x 19.05 cm which equates to an HD video resolution of 1280 x 720 px. Currently the maximum resolution in PowerPoint is 142.24 x 142.24 centimeters or 56 x 56 inches. Which means the maximum standard 16:9 video resolution would be 5K (5120 x 2880 px).
These resolutions are only relevant to bitmap graphics as text, shapes and vector graphics will scale will scale to the display resolution. Currently PowerPoint supports .svg and .emf vector graphics.
Using a DPI calculator it appears that Powerpoint uses a print resolution is 96 DPI or 38 dots / cm.
This article outlines how to change the export resolution of a PowerPoint slide.