Know nothing about the Synapse but I do have a Nashbar CR-5. My previous bike a Nashbar CR-2 was a 58. When I compared the geometry of the 2 bikes I decided to go with the 56. If I remember correctly the top tube of the 58 was about the same size of the 56. Thus I decided to go with the 56. I don't know bike geometry as well as many on this site but, it seems the top tube length on the CR-5 is a little bigger on them compared to other bikes of the same size.

The 56 fits me great once I got a 120mm stem. For the record I am 6' tall with long arms and terrible flexibility. The bike itself I thoroughly recommend. Just wait till they have their 20% off sale so you can get the CR-5 for around $1400 shipped.