Lets say your clip looks like this in Premiere…
The frame size of the file is too small for the frame size of the sequence it is in, which is why there is a black border around it. To fix this, follow these steps:
- select the clip you want to resize
- right click and select “scale to frame size”
- make sure not to click “set to frame size”
Your video is now resized to fit in the frame. This also works with clips that are too big in the frame of the sequence.