Was Tang-dynasty Chang'an the biggest city in the world, and what ended its golden age?

By most accounts, yes, for much of the dynasty's existence. Chang'an, the Tang capital, is described as the world's most populous city for much of that time. Around 750 AD Chinese records called it a "million-man city", and most modern estimates put the walled city's population at 800,000 to 1,000,000. Historians generally regard the dynasty as a high point of Chinese civilisation and a golden age of cosmopolitan culture.

Its walls rose 5.5 m high around an area of about 78 square kilometres. Foreigners were part of daily life: two of the largest ethnic groups in Chang'an were Han Chinese and Sogdians, there were at least five or six Zoroastrian temples, the Church of the East first settled in China around 635, and the West Market had a Persian bazaar. The picture was not all welcoming, though, as several laws enforcing segregation of foreigners from Han Chinese were passed under the Tang.

The Tang is traditionally considered the greatest age for Chinese poetry, with Li Bai and Du Fu among its most famous poets.

The golden age cracked in the mid-eighth century. After reaching its height under Emperor Xuanzong, the dynasty was hit by the An Lushan rebellion (755 to 763), which ultimately destroyed its prosperity. The court fled Chang'an, and Uyghur forces helped recapture the capital but refused to leave until they were paid an enormous tribute in silk. Agrarian rebellions later ended the dynasty in 907.