The smallest I know of is Movie Maker's smallest dialup option, which give a 160 x 120 picture size. However, the picture is not very clear, and any text will be blurred somewhat. I would suggest making a few using several resolution formats and finding the best "happy medium" for size and quality.
I need to send clips by email and want the 'lightest' format? What is the best to convert clips to to send them by email, smallest file size?