Query: How many kilometers was Qing China at its height?
Context: Qing China reached its largest extent during the 18th century, when it ruled China proper (eighteen provinces) as well as the areas of present-day Northeast China, Inner Mongolia, Outer Mongolia, Xinjiang and Tibet, at approximately 13 million km2 in size. There were originally 18 provinces, all of which in China proper, but later this number was increased to 22, with Manchuria and Xinjiang being divided or turned into provinces.