In Powerpoint, if you want to change the slide size, you go to the "Design" tap, and then click the dropdown button for "Slide Size."
It only gives you two seize options: 4:3 and 16:9, and then "other sizes..."
95% of what I do is for print, and 95% of that is letter. It's annoying that I can't "pin" 8.5"x11" as one of the standard options. It takes 5 clicks to change the slide size every time I start a new file, unless I remember that I have a blank 8.5x11-sized document saved as a sort of work-around.