55.5 - I am 5'10" with a 32 inseam. There was a slight misunderstanding on the frame size, (I ordered through On*One at first, but my order was re-routed to Zedsport - the short story is it worked out perfectly, just had to be patient), but a man from the Zedsport facility called to double-check before they shipped - otherwise, I would have ended up with the XL, which is 57.6cm TT.
Like I said - good folks at On*One/Zedsport