Setting Project Aspect Ratio
Before you start working on your project, you should set your project’s aspect ratio. The default aspect ratio is 16:9, which fully utilizes a widescreen TV or display device, as well as YouTube’s full video window. You can also revert back to 4:3 if required, just ensure that the aspect ratio of your media matches your project. To modify the aspect ratio for your media, see Adjusting Video Clip Aspect Ratio.
To set the aspect ratio for your project, click 
> Edit > Aspect Ratio > 4:3 or 16:9. The preview window changes according to the aspect ratio you select.