I'd agree with Robert. Try to compare stack and reach along with effective top tube. I had a small Ridley that I thought would be perfect given the numbers (and I really wanted a 140mm head tube), but in the end the top tube was simply too long and I had a hard time getting setback, and good stem fit. I loved the Ridley but just could not get it to fit the way I wanted. Should have gotten an XS.

Test ride would be nice for the basics of course, just understand that if you buy it they can make basic changes like stem length.