What model of camcorder are you shooting your video with and what format and resolution is your video?
What are your project settings (as listed under Project Settings, under the Edit menu)?
What Export & Share resolution and format are you exporting your finished movie as? When you watch this video on a good video player (like the excellent, free VLC Media Player) does it have a black frame around it?
These three factors are the main factors that affect how well your video fills the video frame.
I use a Canon Rebel t3i. I have tried exporting it with 720p and 1080p. Both seemed to not work. I just recently tried exporting a video again, and it seems to work. I did lots of different tests with different audio and videos just to experiment with the black border problem. Im starting to think that its the audio that's effecting it. It works perfectly fine with some audio, but then a few different audio files, it appears with the black border. I might be wrong but its just a thought haha. Any help is still appreciated though. What do you suggest the project setting and export and share format should be? Thanks!