There are way too many variables to adequately cover this in a simple forum posting. I'll tell you what I ride and why, maybe that will help you determine your needs.

I'm 5'10ish with a 33" floor to pubic bone measurement. My main road bike, a Surly LHT is a 56cm frame, but the fixed gear frame I just ordered is a 58cm. Why? Surly frames tend to have longer top tubes for a given size. My LHT has a 570mm effective top tube, but the fixie, while a 58cm, also has a 570mm top tube. Different "size" bikes, but same effective top tube. If the top tube is the same length between two models, I prefer the larger frame to reduce saddle to bar drop, but you may want more drop and prefer a smaller frame if the top tube measurement is the same.

So, we have three variables here: Standover, top tube, and fit preference. Standover is easy to determine by measuring from the floor to your pubic bone. Top tube length is easy to measure, but hard to determine what is right for you unless you know from experience. Fit preference is even harder to determine for a 3rd party. I prefer a more upright riding postition (as shown by the stack of headset spacers on my bikes), but you may want something lower.

What do you ride now? What are the frame dimensions? What are your dimensions? What do you like/dislike about the fit of your current bike(s)?

Chris