I'm 5'6" (on a good day with the right shoes on ). I got my Giant in a small. Ride them all and whichever one is most comfortable, that that one. But I will add that you won't truly know until you've gotten to put some miles on it. I've changed just about everything on my bike. From stems/bars/seatposts to shifters/and deraileurs. It's perfect now. You don't have to do this, I did because I enjoy fiddling with the bike. The bike was fine from the start except I felt like I was sitting too far back. A change to a seatpost with no setback fixed that right away, but I changed the rest of it for too many other reasons....

For comparison, my partner rides an FX 7.4 in a medium sized frame. He's 5'10. But when I ride his bike, it feels great to me. The standover height is about the same as my Giant, but the top tube is shorter but the stem on his bike is longer. I run a 90mm with a 10 deg. rise. His stem is 100 mm with an 18 deg. rise.