This depends on how busy your kitchen can get and how much food you like to cook at once. The bigger the capacity, the more flexibility you have to cook several dishes or courses at once. Cookers have two cavities. One is the main oven and the second is a smaller cavity that could be just a grill or could double up as a second oven. Capacities are measured in litres. Anything with a 65 litre capacity and above will suit a large household or budding chef.