Is King Khalid International Airport in Riyadh actually very big?
It depends how you define the airport. I don't think your Google Maps method is sound, however; it depicts built-up areas, not the airport boundary. You couldn't draw the same map around LAX, say.

The standard for measuring the area of an airport appears to be the total land owned by the airport. And jet age airports, with their expansive noise buffers and office park reservations, tend to take as much as they can get. DEN is reckoned the largest airport in the U.S., at 33,531 acres (~136 km²), though only a fraction of this is developed.

The numbers for total land are easy to find, the numbers for improved land less so, but I found an informative Globe and Mail article about Montreal-Mirabel. When originally created, the land seized for the airport was larger than the city of Montreal itself— around 97,000 acres (~392.5 km²). The article notes that the airport perimeter itself was only about 5,200 acres (~21 km²). Still, it's the former number that is commonly bandied about, and the measure by which Mirabel was considered the largest airport in the world.

According to Kable, a London-based business information firm, King Khaled International Airport covered an area of 225 km² when it opened in 1983.

In 1999 it was surpassed by King Fahd International Airport, which occupies 776 km². This is roughly the figure given by the 2013 Guinness Book of World Records.