2017 Giant TCX medium has ETT =545mm, but head tube is only 145mm, which makes it feel quite low.

I recall seeing an equivalent frame size on one Giant geometry chart of 52.5 (seat tube length)- which sounds very small to me compared to other manufacturers. It’s not a good measurement of actual frame size IMO - better to use stack and reach.

Had I had more experience, I would have have chosen the M/L size, which is a little closer to a typical 56cm frame. My other bike is a Spec Roubaix in 56cm and this fits well - although I can’t get the bars low enough on this bike because of the tall head tube!