The circumference of my head is circa 57cm, so if you're within 5 or so either side of that it ought to adjust to fit you.
I will measure my head when i get home. I have a mk6 medium helmet and it dont fit because I have a massive head.
I'm not sure about that 50-60 cm thing. My helmet is an Extra Small one (I had to wait for it to be delivered specially when I started training) and I wear a 53cm Beret