Every now and then I put together a video with PowerDirector and upload it to Youtube. Occasionally I have issues with 16:9 videos having letterboxing and the video is squished into the remaining space. In the past I’ve reuploaded them with the proper settings but that uses a lot of bandwidth. Today I discovered a new way to fix it without having to delete and reupload. Just Edit the video and add a new tag called yt:crop=16:9 and save the video. After that it will automatically crop the video to the right size!

Source: YouTube Help