2 runways, 3 terminals, room for expansion for 2 more terminals. Capacity for 27 million passengers per year, going up to 50 million once expanded.
Compared to the absolute largest airports in the world (around 100 million passengers per year), yes, it's "medium". But it would be in the top 10 airports in Europe and still something like top 50 in the world, over time.
Right now it's still only France, but German politicians are talking about adopting a similar strategy.
The French model actually aims to ban flights that have a sub 4h train connection. Meaning the Eastern European nations without high speed rail would have fewer flights affected if this becomes EU policy.
It would cut a lot of flights to neighboring countries, and domestically in Germany.
Quite simply, if you're under 4h train connection you're far enough to justify a flight.
BER is pretty safe on flight numbers.
A railroad investor could look at existing flights and accurately project how many passengers they can expect, since flights would be blocked the second they open their carriage doors.
According to [1] and [2] it wouldn' even make the top 50.
As a German I feel insulted by the whole affair. It's ridiculous how incompetent and corrupt this project was managed.
1: https://gettocenter.com/airports/top-100-airports-in-world 2: https://en.wikipedia.org/wiki/Berlin_Brandenburg_Airport