Owning both a small and a large slow cooker is recommended if the number of people served often fluctuates or if the types of foods prepared vary widely. A slow cooker should be 1/2 to 3/4 full, so the appropriate size depends on how many servings are needed.
A 3 1/2-quart slow cooker feeds two to three people, a 4-quart unit feeds four and a 5-quart device feeds up to six. A 6-quart cooker is recommended only for large families or for individuals who wish to cook a large meal and freeze the leftovers. Having slow cookers in multiple sizes also allows cooks to prepare a greater variety of food. A 2-quart cooker is just the right size to make oatmeal overnight, while a larger unit is required for a roast.