The size of the patties should be determined by the size of the bread you will be putting them on. Also remember that the higher the fat content of the meat the more the patties will shrink as they cook. If you are using 85/15 lean/fat or higher, make the patties larger than the buns, otherwise the same size as the buns will work best.
Another thing to be aware of, is that all the patties to be the same size so they cook at the same rate. If not, you will end up with small overcooked burgers and large undercooked burgers. By taking a ball of the ground meat and holding it in your hand you will get a feel for the size and be able to make all the burgers the same size.