Interesting discussion. I just purchased a shiny new Salsa Vaya with sloping top tube and fitting was foremost on my mind . The effective top-tube is 56cm(22"). The seat tube is only 53cm(21") and 6" of seatpost are exposed.
Hanging beside it is my 1976 Excalibur with horizontal top tube. The seat tube measures 58cm (23") but the top tube length is only 56cm(22"), same as the new bike. Only about 4" of seatpost is exposed. Both fit my 5'11" frame perfectly.