It's not possible to change the size of your slides in PowerPoint Online. However, if you have desktop PowerPoint, click Open in PowerPoint and change the slide size to Wide screen (16:9), Standard (4:3), or a custom size.
Follow the steps in Change the size of your slides.
When you save the presentation in PowerPoint, you'll see the new slide size the next time you open the presentation in PowerPoint Online.
Tip: We're continually updating PowerPoint Online based on input like yours. Tell us how we could improve this feature for you. At the bottom of the PowerPoint Online window, click Help Improve Office.