There's no definite and precise answer to that. As far I can tell, St. Petersburg comes the closest. Built literally from scratch, on desolate marshlands on order of Russia's greatest Tsar Peter the Great to signify Russia's rise to the status of empire, it reached significant population of nearly 200 thousands in 1760, by that time, London nearly had almost four times larger population, but at the same time, it had benefit of existing for few centuries prior to that.

During the population boom of Industrial Revolution many relatively small cities expanded rapidly, and this could be a good answer to your question. Machester is a textbook example. At the start of the 18th century, Manchester was a small, market town with a population of fewer than 10,000. By the end of the century, it had grown almost tenfold, to 89,000 souls. In the 19th century, the population continued to grow unabated, doubling between 1801 and the 1820s and then doubling again between then and 1851, to 400,000 souls. This was phenomenal growth transforming Manchester into Britain’s second city.

After WWII, Warsaw (much like Stalingrad) was effectively destroyed, with over 90% of it's buildings razed to the ground and population that was either killed or fled the city, leaving only 150k inhabitants.

Hope I satisfied your curiosity :) Decade after the war, it was a city with one million inhabitants.