The aspect ratio of a PowerPoint slide is the ratio the width makes with the height. For example, a slide that has a width of 4cm and a height of 3cm has an aspect ratio of 4:3. In versions earlier than PowerPoint 2013, slides were more square (for example, 4:3). However, much of the world’s TV and video display has moved to widescreen and HD formats, and so has PowerPoint. The default slide size is now widescreen (16:9).
But you can change the size of your slides.
Go to the Design tab and click Slide Size (in the Customize group). Choose either Standard (4:3) or Widescreen (16:9) for a quick change.
Because changing the side size affects the amount of space available, PowerPoint automatically scales your content. When PowerPoint is unable to automatically scale your content, it will prompt you with two options:
You are not restricted to aspect ratios of 4:3 and 16:9. You can actually specify your own slide dimensions. Go to the Design tab and click Slide Size > Custom Slide Size.
You can use the Slide Size window to specify the precise dimensions for all the slides in your presentation.
There is no option in PowerPoint 2013 to change the default slide size. However, you can change the slide size in your current presentation, save it as a theme and then make that theme the current theme. I know, this “solution” reeks of after-thought.
This is a poor solution if you don’t want other aspects of the design to become default. You might be better off creating a new blank design with your preferred aspect ratio and then making that theme the default.
So, change your slide size following the directions above. Then click the More button in the Themes group > Save Current Theme.
Name the theme and save it. Then, in the Theme gallery right click on the theme you just save and select Set as Default Theme.
Now when you create a new presentation, it will have your new theme and the slide size you gave it.