According to the Specialized bike size guide on their website, you on the cusp of 56cm and 58cm. Like you I am 6ft. I have a 56cm S-Works Roubaix and it fits me perfectly.
Or you could follow their advice from a FAQ on the Specialized website:-
"How do I know the frame size I need? how I compare sizes: M, L, with inches: 18", 19" etc.?
Answer
The best way to figure out what frame size that you need is to go to a specialized dealer, and have an experienced shop professional work with you.
You need to sit on a few bikes, ride a few, compare our bikes with what you currently might be riding, and if you think your current model is too big/too small. And factor in your riding style & experience.
I have always believed that the most important measurement is not M/L, or even 18/19. The crucial number is how "long" the bike is, and that is the horizontal length of the bike from the center of the head tube to the center of the seatpost.
You can get that info from your current bike, and compare it to the specs we post online in the Bike geometry. Also factor in the length of your stem as well, as that affects the "length" of your bike. Being on a bike that is too long or too short sucks. But if it is just right, it is a wonderful thing, a bike that your body is comfortable & centered on will be stable as well as agile.
good luck!"