If you're unsure of your sizing, or unwilling to take a bit of a leap of faith, you may want to consider purchasing from an LBS. It sucks being on a bike that isn't your size, especially when you're stuck with it.
That being said, I sized myself and bought online. Make sure you run a few different fit calculators to find out your general frame size.
I'm 5'10" with a 30in inseam and ride a 56 Specialized Allez
I just bought a Fantom Cross Uno in a 56 also after test riding the 54 at the shop and finding it to be too cramped.
I bought it from Cycle Spectrum in Orlando FL. They are a chain of shops that sell BD bikes with the positive that you can actually try the goods out before making the purchase which was why I didn't mind paying the sales tax to be able to go out and test ride the bike first instead of gambling with BD.