I'll go the opposite of conventional wisdom: my roadie is smaller than my CX.
I bought my 57cm CX (Felt F1X) w/o having much of a clue about what size I was. Picked up a 56/54cm (top tube/seat tube) Cannondale R800 for a quick road bike.
Both work. Since the road bike is smaller, it feels even quicker to handle. I put a short (85mm) stem on the CX and it feels fine.
I ride almost exclusively on the tops, not on the hoods or drops. Cross-top brake levers essential for riding in traffic (Seattle). I even put 'em on the roadie.