Click "Pages" from the right-hand panel to expand the Pages panel. Alternatively, press "F12." Double-click the left side of the "A-Master" page that is located in the top of the Pages panel.

Continue to drag the "Master Page" icon over each page you want to use with the selected page size. Repeat Step 4 to select a new page size and then apply it to other pages within your project.

You can add page numbers to your PowerPoint slideshow through the "Slide Master" page, which lets you edit your entire slideshow at once. Once you add page numbers to your PowerPoint slideshow ...