Can't export the full video

The image of the issue: https://imgur.com/a/17DdFYq

Hi, please refer to the asserted link which is the screenshot of the issue I've met with. I don't know how to alter the width of the 'bright' part of the video so that I can export the full video and I tried to sort it out using Google search but there's no helpful info about this.

This issue is driving me crazy and a timely reply would be appreciated.

Thanks,

Wei