Size
You need to look at both the internal size and storage capacity as well as the overall dimensions of the cooler. While a large esky might seem ideal to take on a camping trip so that you can bring as much food as possible, the esky will be useless if it doesn't fit in your car.
Larger eskies typically keep food colder longer because they can hold more ice, but they cost more than small coolers. Eskies also work most efficiently when they are fully packed, so choosing a size that is right for your needs is important.