Spent the past 2 years on a 53cm Lemond. Let's just say "too big!" Over the past several weeks, I test rode many bikes, from 48cm - 52cm, including Treks, Specialized, Cannondale, Orbea, Felt, .... you get the drift. After the 2 years with the Lemond (my first real road bike), I now understand what fits and what doesn't.
Anyway, I ended up with a 50cm Cannondale R1000. I almost went for a Felt F2C which was also 50cm, but it just wasn't right for some reason. The next best fit was an Orbea Dauphine (sweet riding bike, and also 50cm). Didn't dig the paint, so passed on it.
I also rode a 48cm Cannondale, but it was too cramped for me.
So while the 50cm C'dale was right for me, that didn't mean that every 50cm would fit the same. I would suggest that you target a range and ride as many as you can. Some will be off the list quickly, while others may be harder to decide on. For example, I probably spent 2 hours riding the C'dale and the Felt back to back before I decided on the C'dale (I was trying to convince myself that I could make the Felt fit).