the Sarthe is a great bike, I would say 56 might be one size to small...but people vary greatly so definitely have a shop size you and switch out stems and seatposts (different setbacks and what not) and you'll be golden
assuming you actually are 6ft. the 57 would look like a good option for you assuming certain things. the next size down is a 55, which unless you have no flexibility isn't going to be a proper choice for you. go ride a 57. i'd be utterly surprised if you didn't fit it length and height wise.