Did a search and found a few threads with sort of the same question, but no answer.

I've always been on traditional frames (56cm). Test rode a friend's 56cm SL2 today. Is it essentially the same size as say my 2006 madone? It felt slight bigger, which may be because of the bigger head tube.
Is the length of the seat tube (i.e. 56) the important measurement?

Maybe I just need to get used to the thicker construction and the longer head tube??