To be considered a desert, an area must receive than 250 millimeters of annual precipitation. So, in literal terms and in terms of sheer size, the Antarctic Desert is the largest desert on Earth, measuring a total of 13.8 million square kilometers. Antarctica is the coldest, windiest, and most isolated continent on Earth, and is considered a desert because its annual precipitation can be less than 51 mm in the interior.

Source – https://www.universetoday.com/27064/what-is-the-largest-desert-on-earth/