Well, Elon never said it was a plan. He just calculated that there are about 750 watts per square meter at the latitude of the middle of the US. The solar panels are about 20% efficient. So 1/5 of 750 is 150. The US needs about 1300 gigawatts of power. So divide that by 150 and it gives the square meters needed. Take the square root of that and it gives a square of land about 100 kilometers on each side. Then multiply that by 1.6 kilometers per mile, just for a safety “fudge factor” and it gives 100 miles on each side. This estimate is very rough, it’s not really a plan. Pieces of the array would be spread across the whole US to make them closer to where the power will be needed.