To crop the size of a video, you can follow these steps:

1. Open the video you wish to crop and click on the Edit option.
2. Click on the Crop Button.
3. Click on the “Aspect Ratio” button on the right top corner of your screen.
4. Select the aspect ratio you wish to crop your video to.
5. Move the video to frame it and pinch in and out for the right kind of zoom.
6. Change the background color if needed and specify format for the output file.
7. Click the "Export" button.

Alternatively, you can use a media player like VLC to crop the video.