At 5'11", you can probably use any of the standard sizes: 175, 172.5, or 170 mm.
You might stop a bit and figure out if you tend more toward the "spinner" or "masher" end of the spectrum. If you're a masher, the 175 gives you an extra quarter inch of leverage. A spinner, you save that quarter inch to get extra leg speed. Not sure, split the difference, 1/8" toward each.
If you're obsessing over that 1/8", either you're OCD or know enough to make the decision without our inputs...