Bike size - will a 2cm make a difference
Many people fall between two sizes and can get a smaller size or larger size to work, possibly with a few adjustments/parts (e.g. new stem, etc.). 

You really have to try the bike to find out. I happen to be 5'11" as well, and my neighbor (who is also 5'11") did own a Synapse 105 (a few years ago; doubt its the same bike still) 56 cm and found it fine but on the bigger side. I found it a bit small when I tried it. 

Depends on the rider -- there is no way to find out otherwise without trying them.