Originally Posted by Iride01

I'd disagree with going to a larger frame unless the OP wants to be more stretched out with arms extended. On old style and very oversize bikes for my 5' 11" and 34.5" inseam, that approach worked well to make me comfortable on a 25" (64 cm) Schwinn Varsity. However on newer style geometry and my own personal desire for a low and more aero position I'm finding that smaller frame sizes seem better.

Though one does have to prefer that larger saddle to bar drop that a smaller frame will give. If that isn't desired, then a different model bike with a higher stack in a smaller size might be the thing to look for. So if I was to go a size smaller from my 56 cm Tarmac, then I might consider a 54 cm Specialized Roubaix which will put my bar drop from saddle about the same as it is currently on my Tarmac.

I'm wondering what year bike the OP has since for quite a few years Giant has given the size of their TCR in letter sizes. And on their geometry charts they don't give a measurement that would let us easily equate to the 58 cm size they are stating.