In earlier versions of PowerPoint, slides were squarer (4:3). Much of the world’s TVs and video has gone to widescreen and HD formats, and so has PowerPoint. The default slide size is now widescreen (16:9). Here’s how you can change the size of the slides:

To change the slide size from standard to widescreen, do the following:

1. Click the Design tab, and then click Slide Size.
2. Click Widescreen (16:9).

To change the slide size from widescreen to standard, do the following:

1. Click the Design tab, and then click Slide Size.
2. Click Standard (4:3).

 Note:    When PowerPoint is unable to automatically scale your content, it will prompt you with two options:

• Maximize: Select this option to increase the size of your slide content when you are scaling to a larger slide size. Choosing this option could result in your content not fitting on the slide. 
  
• Ensure Fit: Select this option to decrease the size of your content when scaling to a smaller slide size. This could make your content appear smaller, but you’ll be able to see all content on your slide.

Note: You can also change your slide size to a custom dimension. To do this, on the Design tab, click Slide Size, and then click Custom Slide Size.

In the Slide Size box, set the height and width dimensions, and the orientation.

Set 4:3 aspect ratio as the default size for your slides

If you want to continue to use the 4:3 aspect ratio, you can set it as your default slide size. Here’s how:

1. With the 4:3 aspect ratio applied to your slides, on the Design tab, in the Themes group, click More , then click Save Current Theme.

2. In the File name box, enter the name of the theme, and then click Save.
3. Right-click the saved theme, and then pick Set as Default Theme.

Source: Microsoft