You can do this on the command line using ffmpeg and the crop filter. Handbrake is a great tool, however it uses the same ffmpeg libraries that you can use directly from the command line.
The basic command is as follows:
% ffmpeg -i input_filename -filter:v "crop=w:h:x:y" output_filename
w = width of the output video
h = height of the output ...
In my Macbook Pro (High Sierra), I did:
Open "iMovie" application
Click on "Create New"
Click on "Import Media"
Choose my video and click on "Import selected"
Right click on video preview, click "Select entire clip"
On the right, where the tool menu is shown, click the third icon