I won't repeat what has already been said except recap that and add that three ways exists to measure size by seatpost - center of bottom bracket to center of top tube, center to top of top tube, and center to top of seat tube (Fuji is one that did this a lot until recently). Sloping top tubes throw this off but practically all manufacturers include data on the virtual seat tube size by extrapolating what the size would be if the top tube was horizontal. If that isn't availble, you can approximate the size by using the top tube length since the majority of bikes in recent years use a "square" methodology in sizing (the top tube and seat tube lengths are the same).