Handbrake has some presets for video and audio settings which are useful for encoding for specific devices or for specific types of video.
To test this out, after you have selected your source and destination file, select the "iPodLowRez" setting from the right of the default screen.
You will see that this sets a size automatically and if we click on to the video tab we'll see that a default bitrate of 700 kps
is applied too.
These presets are useful for preparing video for an ipod.