Change sets the destination folder for the screenshots.
Video & audio, video, audio option buttons set what is to be recorded. Video format is Webm, and although video size can be changed, the aspect relationship of the screen cannot. I had to make many tests to find the right point between quality and real time encoding speed, I think I managed to do so, your mileage may vary, as a point of reference my developing machine is an AMD A10-7870K without discrete GPU and can record full hd video without any problems.
Full screen, Half screen set the width and height of the recording.