Normal population would have been about a quarter million in the 1920s, but refugees from the war pushed that up significantly. Multiple sources agree on this as the pre-surge population. Post-surge, I've seen the number at a million and i've seen it at 600,000, with claims that half the population was wiped out in the massacre. 600,000 is more reasonable. The official Chinese numbers are that 300,000 people were killed.

This is from a party-sanctioned source, and the numbers can be taken as official from China's point of view:

Zhang Lianhong, professor of Nanjing Massacre Research Center in Nanjing Normal University, published an article in Beijing Daily, in which he used historical fact to show that the population of Nanjing urban area was between 367,000 and 467,000, and the overall Nanjing population was between 535,000 and 635,000.

This would include refugees.

If the population was normally anywhere under 300,000, and the official population was 600,000 or more by 1937, and if the official number of deaths is accurate, than you could argue that the population never dropped below the pre-massacre number given the number of refugees going into the city.

It's really hard to get accurate numbers, of course. All sides have likely fudged the numbers. But I'd argue the ~600,000 number should be pretty accurate for that time given that those sorts of records would have been well kept. I'll need to do some actual digging to get number from the early 30's.