Are you cooking for yourself or multiple people?
4 quarts is about the minimum size needed to cook a meal for a family of four. 6 quarts is the most common you'll find in the larger units, and with that you can cook a few days' worth of food for a single person. Also, most slow-cooker recipes are setup for the larger units.
1.5, 2, 3 and 3.5 quart units are also available, but tend not to have the added features, like a timer, automatic temperature switching or removable dish.
EDIT- Crock Pot's Smart-Pot 4 quart digital is a good option.
If you need the extra capacity, go ahead and get a 6 quart version.
If you want something smaller, this 3.5 quart Cuisinart is the only thing I could find under 4 quarts with digital controls.