Giant sizing is like Small, Medium, Large. Very limited really. The size that was right for me had a top tube 2cm too long. Cannondale on the other hand makes the Six13 in 12 or 13 distinct sizes. I'm riding a 56cm. Fit me like a glove.

So it goes. If it fits, it's great. If not it's not even on the radar screen. Custom bikes? That's the other route. The pain in the ass expensive one. Not for me. The C'dale fits me like a glove (56cm size, 56cm effective top tube length). I'm 5'11, 34" inseam, relatively short torso length. In fairness to Giant their sizing would fit a majority of riders' phsyiques. Short torsos/long legs are less common than more balanced proportions or riders with long torsos/shorter legs.