100 miles on each side
Area Required to Provide 100% Solar Concentrator Electric Energy for the USA
Square with 100 mile sides:
10000 (square miles) = 2.6 × 10^10 m2
Each meter squared gets you 1000W of solar power.
This gives about 2.6x10^13W, or 26,000 GW.
Assume that conversion efficiency is 10% to electricity using solar concentrators (Ausra claims 30% efficiency). Divide that by 4 because you have sun 6 hours of the day, so you have 26,000GW/40 = 650 GW, or about what we need to power the USA.
The USA has 3.5 million square miles. 10,000 is about .3%.
Comparison to Roads
There is about 4 million miles of roads in the USA. The average width of a road is 50 feet, or .0095 miles.
This constitutes a total of 38000 sqare miles of roads - or 4 times as much road area than the area required to go 100% renewable on solar energy via solar concentrator electric power.