Germany has had an absolutely terrible century demographically. They had 78 million people in 1970, but only have 83 million today even after getting sizable immigration. Without the territory loss and war dead they probably have 100 million by 1970 even with a collapse in fertility relative to the rest of Europe (as per OTL).
Germany had terrible Post War fertility, with a weak baby boom and weak fertility even compared to the rest of the First World ever since. But before WWI Germany had one of the highest fertility rates among developed European countries.
(I lost the pre WWI German TFR, but here is net reproduction rate instead)
I would imagine that without the war Germany remains more traditional, or atleast more traditionalist for longer. Nationalism and Ethno Nationalism won’t be as discredited, religion might be more prevalent, their government remain Autocratic, and the idea of direct competition between Western nations will likely be more prevalent. Germany could potentially buck the trend of sub replacement fertility and maintain fertility above the replacement rate until modern times, or for substantially longer.
Mostly the same for Germans living in Austria-Hungary.
If Germany has fertility that goes as poorly as OTL, probably around 100 million people in Germany’s 1914 borders. But if Germany is able to support large families as effectively as Israel to the present, they would have 200-250 million people in their 1914 borders.
I’m guessing Germany would do much better than OTL given that Germany did uniquely bad, but still probably comes well short of modern Israel and eventually fall below the replacement rate. So probably around 150 million people in Germany (who aren’t all German) and another 30 million Germans outside of Germany.