The size is measured in quarts and ranges from 1-11 quarts, so make sure you get the size that's right for you. The smallest size is not really suitable for one-portion meals. Anything from about 4 quarts is considered family size. Sometimes it's not a bad idea to buy a larger size just in case you have guests over or if you want to freeze some meals for later. The largest models with automatic timers tend to be the most expensive.
Power is measured in watts and it ranges from 1500-2000 watts. Larger models usually have more power. Most models offer at least 2 heat settings, but some models offer more then that. It's always better to have an "automatic" option allowing you to set the timer. Another feature that you should look is a "keep warm" setting. This stops food that’s ready to serve from drying out.
Most slow cookers have removable pots that are dishwasher safe. Look for models with stay cool handles and glass lids so you can see what’s happening inside. Make sure to buy the models that are easiest to clean.