I'd say 57. I am 5-10 with long arms and the 53cm Mercier fits well, a little small if anything. I had a 56cm Cannondale R500 before that, and I'm guessing the Trek is sized more like the Cannondale. On the bikesdirect site - they say the 53cm Mercier is equivalent to a 56 (not that there is a "standard" 56 size).