I'm 5'9" and I ride a 54cm my first bike but I think I could be riding a 52 as well if the synapse comes in that size my inseam is 32" the 54 fits me good so far but I do work out so I'm kind of mascular so I guess that's helps to avoid neck pain or back pain so far knock on wood .

What's your inseam? Maybe it's just a matter of getting use to it