Today, there are about 30 megacities in the world, where more than 10 million residents live. The largest is Tokyo, at about 38 million. Recent announcements by the government of China could lead to the worlds' first gigacity (for want of a better term, used here to denote a city of more than 100,000,000 population, see note). According to the Nanfang Insider, the economic integration of megacity Beijing, megacity Tianjin and eight cities (prefectures) in the province of Hebei would result in a city of 130 million. China Daily is a bit more circumspect, indicating that the Beijing supercity would have only 85 million.
The giga/super city would be tied together by new rapid transit lines and highways and surrounded by the 7th Ring Road, adding to the six that have already been built. The 7th Ring Road would consist of two roads, circling most of the area, and extending to a combined 850 miles (2,200 kilometers). By comparison, London's M-25 is 117 miles long (188 kilometers), the Moscow MKAD 68 miles long (109 kilometers) and the Washington beltway is 64 miles long (103 kilometers)
The giga or super city is not likely to really be a city, because it would be much larger than a labor market (this is why the near continuous urbanization from Boston to Washington or Tokyo to Osaka-Kobe-Kyoto is not a city). The estimated land area is 67,000 square miles (175,000 square kilometers). This is nearly as large as Cambodia or the state of Oklahoma. Providing the point to point daily commuting in such a large area is well beyond the capability of any affordable transportation system. Star Trek like teleportation could do the trick. Meanwhile, however, there is plenty to be gained from the economic integration of this large area.
Graphic of the new 7th ring road from nanfang.com:
Note: Technically, a gigacity would need 1 billion people (10 to the 9th power). However, megacities, with their 10 million minimum are also wrongly named. A city with a mega city would have 1,000,000 people (10 to the 6th power). Artistic license justifies the gigacity term under the circumstances. Besides, with the slowing growth of world population, it seems unlikely than any city will achieve a population of 1 trillion.