Bike Size

So I am looking into buying a new triathlon bike, currently I use a bike that is a size larger than I need(passed down from my dad). I am 5'8, and would like to think I have pretty normal proportions. I will likely get a bike fitting after I purchase a bike but I need to decide on a size before I go and get it fit to me. It seems like ill most likely be buying a (used) cervelo P2/P3, and I am curious what people's experience has been with their sizing, I believe that I *should* fit a 54cm frame but I have seen various reviews saying that they fit better on a 51? Just looking for some other people's experiences/ wisdom.