I'm with everyone else. Find a shop that carries them. Make sure they have one in stock before you drive too far, and go get fit. Most shops will order a size in if they don't have it for you to try.

I'm 5'11". When I was test riding tri bikes, I was riding on a 54 p2sl. I ended up buying a QR, but the 54 Cervelo would have worked for me.