Although grills are listed with square footage, included in that measurement are the warming racks, so you are not getting an adequate understanding of the grill's size. A better approach is to stand in front of the grill you are considering and imagine your usual cooking volume. Then ascertain if that particular grill is going to be adequate for you.
The term BTUs will be thrown at you when you begin your grill shopping. Don't panic. The anagram stands for British Thermal Unit and indicates how much heat energy is needed to raise the temperature of one pound of water by one degree F. So if you choose a propane grill, propane has about 15,000 BTUs per pound. If you are considering charcoal, think 9,000 BTUs per pound and pellets about 8,500. A 30,000 BTU gas grill emits 30,000 BTUs in one hour, or about two pounds of propane an hour. BTU's do not indicate how quickly a grill will heat up, but can indicate how much fuel you will be using while you cook.
The average sized grill has two separate burners with two separate controls or one burner with each half independently regulated. Bump it up one notch for four burners. Large grills can include six burners.
How many people do you normally feed when cooking outdoors? Are you outside every night, every weekend, or only when the occasional spirit moves? If you are feeding a larger group fairly consistently it pays to look for the larger, sturdier grill. If it is most often grilling for two on a Saturday night, a two burner might do nicely.