Hi,

My first post here, but I couldn't help but offer my two cents (about what it's worth!).

Did your LBS fit you to a size medium without trying a small first? I'm 5'7" and fit bikes with a top tube of anywhere between 53cm to 54cm coupled with a 100-110mm stem. I'm also leggy (80cm inseam) and ride with a fairly flat back and stretched out position. If I'm not mistaken, the top tube on a medium Giant is 55.5--way too long even for me. I don't know why you would have been sized to a medium unless you had issues of not enough saddle setback or the need of a taller headtube, etc. Riding a bike that's so big that you have to use a very short stem (<80mm) and shove the seat far forward to obtain a comfortable reach can adversely affect your weight distribution and create handling problems.

In any event, I hope you resolved your fit issues and are out there riding and having fun