My Modern sloping top Basso is a 56cm and I'm 6 foot 1", even then it could be a couple of cm smaller and be made up by saddle and bars up.
Way different to my steelies from the 70's which were 23-24 inch LOL
That's a really interesting comment Shaun.
I'm also 6'1" and I have just built up my first road bike in over 30 years.
The bike's a 1999 RSP 853 in 56cm and it fits really well without miles of seat post exposed. I then see 23" plus frames for sale, and wonder what kind of giants rode such bikes.
My feeling is that the more modern carbon and sloping top tube type frames are designed to be a lot shorter in the seat tube and have much more of the seatpost exposed (like mtb's), this is where the length of the top tube becomes all important, due to the reach.
I ride 17.5 to 19" mtb's, but they must have a top tube of at least 22.5" or I end up either hunched up (back ache time), or with an overly long stem, which pushes me too far forward (arm / neck ache time) and makes for a pretty unpleasant ride.