It'd be slightly larger, but not by a ton. While they definitely wouldn't screw up as badly as Mao did (no preventable famine that kills 50 million people, for example), the Kuomintang were not saints by any means. No one child policy means that the Chinese birth rate doesn't drop as fast as IOTL, but it'd still drop with industrialization and urbanization. Taiwan has a rock bottom fertility rate of 1.1-1.2 as of 2021, and while I don't think a fully Nationalist China's TFR would be that low, it would likely be quite a ways below replacement level by 2021. So, with all that in mind, I predicted that Kuomintang China's population would be around 1.4-1.6 Billion, higher than OTL but not drastically.