Many codecs compress to a specific amount of data per second, known as the bitrate. The size of the output file will be dependant upon the length of the video and the bitrate only. Use a lower bitrate and you get a smaller file. There is a trade off, of course; lower bitrates result in lower quality.

When you render to formats like WMV, MOV, MPG, DivX, Xvid, there are various templates available that offer different bitrates. If you want more control then in most formats you can click the custom button and choose your own bitrate.