After a few talks with the owner I found out that he measured the seat tube from center to top which means that the frame size is 56. So, as I'm 182 centimeters (5 feet 11 21/32 inches) high I think that the frame is a bit too short for me. I'm not so keen on buying the bike anymore so my quest for finding the perfect frame continues.

Please don't hesitate to question my judgement regarding the size of the frame. I'm still a newbie.