High, I'm using a hacked GoPro with a 220 lens in 4K mode. Not all the circular image fits on the sensor, but anyway, I would like to crop it (if there is any advantage in doing that). How do I change the size of the crop circle? Its size is apparently fixed...  

Hi! It can be modified by pressing Alt+Drag while in the calibration view. You can check this tutorial for mor info https://vimeo.com/217972391