According to a graduate thesis from the Technical University of Braunschweig, by Nadine May: area of 254 km x 254 km would be enough to meet the total electricity demand of the world. The amount of electricity needed by the EU-25 states could be produced on an area of 110 km x 110 km. For Germany with a demand of 500 TWh/y an area of 45 km x 45 km is required, which concerns 0.03 % of all suited areas in North Africa (BMU, 2004b).
That's just 158 miles x 158 miles (c. 25,000 square miles) to power the whole world.