when whatsapp type is chosen,reduce result to 16mb

tried to cut a video which was 75 mb long. selected whatsapp as end result. The program rejected,
saying that whatsapp will accept only 16 mb. Not very useful.
The program should downgrade the quality of video so that the resultant file is 16 mb or less, so that
whatsapp will accept it. cutome at present needs spoon feeding.