Population size - Country rankings:
The average for 2017 was 38.19 million.The highest value was in China: 1386.4 million and the lowest value was in Tuvalu: 0.01 million. Below is a chart for all countries where data are available for: Population size.
Population size, in millions, 2017
(million, Source: United Nations Population Division, TheGlobalEconomy.com)