Hi Robert,
I'm 6'1" (though I have a relatively long neck so my 'effective' height is probably 6'0') and I find the size perfect. My previous bike was a Lapierre Spicy in medium and that felt a bit cramped. In fact I used a large loaned Spicy for a while, which I much preferred. The measurement you've asked for is 505mm though the positioning of the saddle on the seatpost could allow for an increase or decrease in that figure. I guess you're a fair bit taller than me, or have a much longer inseam, as my maximum saddle height (my peddling uphill height) from the centre of the BB is 720mm.
Hope that helps. BTW if you see me in Surrey Hills, stop me and you can check out the bike for fit.
Cheers,
Andy