You can change the size of a slide in your PowerPoint presentation through the "Design" menu on the Windows PC, Apple MacOS, and web-based apps.

To change your PowerPoint slideshow orientation from horizontal to portrait, you must access the "Slide Size" feature under the "Design" tab.

Open PowerPoint. Click the Design tab and click the Slide Size button in the Customize group. In the Shortcut menu, you can choose to select Standard or Widescreen or Custom Slide Size.