Most ppl think the difference from one size to the next is night and day. The truth is that the top tube difference between a 54 & 56 is about 1.5cm. Go measure out 1.5cm just to see how little that is. Considering most saddle rails allow for 7+cm of fore/aft positioning, not to mention being able to pick up a seatpost with setback, and stem lengths typically have a 3cm range (90mm-120mm), excluding the different types of handlebars and hood positions, the most important part will be the fitting process rather than the size of the frame. A decent fitter could probably get you comfortable on a 54, 56 or 58. Just judging by your height, I'd say you were a 56 or 58, but I'd urge you to take a look at fitting link that I posted earlier. It's far more comprehensive than any of the conjectures you'll find in bf.