What is the largest desert in the world?
The Antarctic Polar Desert would be the largest.
How big is it in size?
It's area is estimated to be 13.8 Million square kilometers.  It is equivalent to a circle with a radius of 2100km. 

This ranks it among the largest land structures on earth. Comparable entities are:
* ~0.8 x of the total area of Russia
* ~1.4 x of the total area of Canada