Size and shape
Slow cookers can be oval-shaped, round or rectangular. The smallest slow cookers start at around 3-4.5 litres and will be more than adequate for singles and couples. If you're cooking for four or more people, you'll want to consider models of 5 litres and above. The biggest slow cookers top out at a capacity of around 8 litres.
Make sure it can fit in your kitchen cupboards or on your benchtop and check the specs sheet for a list of the cooker's physical dimensions.