PowerPoint 2010 and earlier has a default (standard) screen size of 4:3. PowerPoint 2013 changed that to make the default (widescreen) screen size 16:9. In PowerPoint, you can change the aspect ratio of your slides at any time. Keep in mind that everything in your presentation slides will change size and look distorted. If your presentation is 4:3 and you change it to 16:9, everything will be stretched wider.
The best way I have found to avoid this is to start a new blank presentation in PowerPoint, make it 16:9 and then copy and paste the contents of each slide one at a time into your new presentation. This can be time consuming, but this process lets you make any changes you need to your slide content without it being the wrong size.
Here is the direct method built into PowerPoint
How to change my slides to widescreen (16:9) size in PowerPoint 2013.
Open a new blank PowerPoint presentation.
Go to the Design Tab
on the left side, click on Page Setup
Set the Slides sized for: to On-screen Show (16:9)
How to change my slides to standard (4:3) size in PowerPoint 2013.
Open a new blank PowerPoint presentation.
Go to the Design Tab
on the left side, click on Page Setup
Set the Slides sized for: to On-screen Show (4:3)
How to move all of my slides to a presentation that has a different screen size.
In your new presentation, add the same number of slides as there are in your previous presentation.
In your previous presentation, go to slide 1
Single-click on an item on slide 1
Hold down the Ctrl key and click the ‘A’ key. this will select everything on the slide.
Now hold down the Ctrl key and click the ‘C’ key. this will copy everything that is selected.
Go to slide 1 of your new 16:9 presentation and click in the middle of the slide.
Hold down Ctrl and click ‘V’ to paste everything into the slide.
repeat these steps for all of the slides in your presentation.
There are Add-Ins available that will automatically resize a PowerPoint presentation between standard screen and widescreen available but they are not free and do not work 100% of the time.