You are probably only choosing between two ballpark sizes and then it will be fine tuning with bars and stem. If you have a bike that fits, the Gunnar site has good info about which of their sizes will work. The tall headtube allowed me to ride a size larger than I would have guessed, so do your homework. Learn enough to double check anything the shop tells you. I have a friend who bought a custom Waterford, totally relied on the shop for fit, and ended up with nearly three inches of stem spacers!