Tech Tip: Change Screen Size in PowerPoint

In PowerPoint, the size of slides can change with just a few clicks of the mouse. The default size in PPT 2013 or newer is in Widescreen (16:9). In older versions, the default size is Standard (4:3). To change from widescreen (16:9) to standard (4:3) or vice versa, open PowerPoint and navigate to the Design tab. Select Slide Size, and choose either Standard or Widescreen. If you choose Standard, you will be given scaling options. Maximize increases the size of the slide content and may result in not all of your content fitting on the slide. Ensure Fit decreases the size of the slide content and may make your content appear smaller, but all content will fit on the slide. 

Knowledgebase Article: http://helpdesk.etown.edu/helpconsole2012/KB/default.aspx?pageid=slide_size