Rent has exploded in Berlin and is continuing to climb in Berlin. Also if you look outside the city center, i.e. in Friedrichshain, Kreuzberg oder Prenzlauer Berg.
As single developer you wouldn't need 85qm, true. As a couple you probably want to have min. 50-60qm. My personal taste obviously, but I have a family with two kids and we need more than that :)