Janina really made sure we were all set nice and cozy in her apartment.
Her apartment is very well located, definitely big enough to host 4 adults. As one previous comment says, one room is a "kid's room", with an elevated bed about 1 meter off the ground. Although not extremely wide nor long, the bed's dimensions were big enough for my wife and I to sleep in it no problem.
I truly recommend staying at Janina's for your sojourn in Berlin!