Short answer: you can't; any compression involves a loss in quality.
Are you making a standard DVD to be played in a DVD player? If so, you're encoding to MPEG-2. Reducing the average bitrate will make the output file smaller. You can probably reduce it quite a bit before noticing the quality loss.
There are host of bitrate calculators to help you set a target bitrate. It's based on duration and disk size(CD or DVD). It's great tool to have when your video is close to two hours in length.