The laws of physics suggest an area about the size of Texas is all the bigger you can make a grid. AC is much better than DC for areas of that range, because you can adjust voltages with a cheap transforms. However if you get too large and feed the grid from multiple places, there gets to be a point in the middle where power from one end is driving the grid negative while the other is driving positive.

You can get around this with DC, and grids do interconnect with DC power, but that requires more expensive equipment.