I'm 6'0'' and ride a 58cm Madone. It fits me pretty well. I would think you'd be better suited on a 56cm as well. I think either would have been fine for me. If I check any of the various fit calculators online I come up at 57cm. I don't think you'll be too far off on either, as you can adjust the stem and saddle, but I think the 56 would probably be the better starting point.