You can change the background of your PowerPoint slides using the "Format Background" feature located in the "Design" tab.

Learn how to format the background in PowerPoint by customizing options such as solid fill, gradient fill, picture or texture fill, and pattern fill, etc.

Learn how to change Slide size & orientation in PowerPoint. Slides are sized for onscreen slideshow. Orientation is Portrait or Landscape.

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.