This is a very easy way to crop your clips to match your video size.
1)Usually when you add an image or video clip to the time-line, it doesn’t fit the video screen completely. Like this image. Notice the black rectangles on the side of the image? We’re going to get rid of those.
2)To get rid of this, click on the Pan/crop button on your clip in the time-line.(top button)
3)It will bring up this window. While in the new window, right click on the ‘F’ crop and select “Match Output Aspect“.
4)In the window the ‘F’ crop should now match the output in your video. (Video size) Which looks like this.
5)Finish cropping and editing. You should now have something like this: