The biggest city in the world is a more complicated question than it appears on its surface. Biggest city can be defined by a number of different criteria. I have provided two figures; one based on municipality in square kilometers and one based on number of people.
The city with the biggest population is Tokyo, Japan at between 33 and 34 million. It is impossible to know with any certainty the exact number of people in a city as it changes from minute to minute.
The city with the biggest municipality is Nuuk, Greenland at 635,000 square kilometers.