...an 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).
http://www.dlr.de/...

That's just 158 miles x 158 miles (c. 25,000 square miles) to power the whole world. It makes one wonder why the hell we aren't doing this already, though the obvious answer is suppression from Big Oil.

H/t iflscience.com:
http://www.iflscience.com/...

Update 1:

Neither this post, nor Nadine May's thesis are in support of actually turning that chunk of the Sahara Desert into a solar array. It is merely an excellent way to visualize how little space we are talking about.

http://www.dailykos.com/story/2014/06/25/1309388/-Solar-Panel-Acreage-Needed-to-Power-the-Entire-Planet-158-mi-x-158-mi?detail=email