I’m 5ft 11 (well a smidge under) and ride a medium Defy. Giant also do a M/L frame between the medium and the large, and even this felt too big. I’d say you’re riding a frame 2 sizes too big for you, and I’d be straight back to the shop you bought it from to have it out with them cos is the crucial issue.
Quick check on geometry, and Medium Defy TT is 545mm, M/L is 560, Large is 575 and comes with a longer stem too I think. I can’t imagine trying to ride my road bike with an extra 30mm in the TT and another 10mm in the stem, it would be like trying to pilot an oil tanker!
And as for how they size, the Medium is a 50cm compact, fits like a 54, the M/L is a 53.5 but fits like a 56, and the large is a 55 but measures up like a 58.