In absolute terms? Probably over 330 million. That's if Germany was the wealthiest, foremost world power, with a high birth rate, and if it had been such for centuries.
Realistically, with a post-1900 POD? Probably more like 150 million, assuming the worst of both world wars were averted. Germany lost about 12% of its population during world war 1 and the revolution, 8% during world war 2 and after, and another 1% in the late '80s. If we assume that these periods of shrinkage can be replaced with periods of stagnance or even growth, we get a value of 148 million. If we look at the Netherlands, they grew threefold between 1900 and 2012, so with a little bit of stretching, 200 million in Germany might be possible, but that's really only if Germany curb-stomps its enemies in both world wars, establishes itself as the king of Europe by 1950, and decides to attract immigrants as a matter of policy.
EDIT: Didn't see you include Austria. We could have an absolute capacity of just under 40 million using Austria's land. That's not realistic, but it is possible. If Grossdeutshland was a third-world country with a massively booming population (like India), this would be possible: total 370 million.
If we assume a similar realistic population figure with the post-1900 POD, I would wager 15-20 million as the maximum there. So the maximum for Grossdeutschland using a post-1900 POD would be around 170-220 million. I'd put the lower figure as more likely, however.