I am 5'9.5" and in seam length between legs is 33 inch. What is the optimium frame size?
Ten thousand expert opinions can't match a single objective test. I'm about your same height and inseam. I have 52cm, 54cm, 56cm, and even a 51. They all fit, and work for me. One manufacture's 54 will sometimes feel like another manufacturer's 56cm. Fit isn't just about frame size. Stem, saddle position, spacer count, all play into it. Ya gotta test ride a couple of different makes and sizes. If you're like me, you're gonna want to write your findings down on paper to review once the "wow, that was cool" factor wears off a bit.