I'm 6' tall and use 140's. I like 'em; they're the same as the ones in this ebay listing, just shorter (and equally cheap):
http://cgi.ebay.com/FMF-ProStroke-Al...2em118Q2el1247
It is true that most bikes come with 170's stock. But it doesn't make sense to me. In the old days, when all bikes were dreadfully overgeared, most bikes came with 165's. Now that we habitually spin in lower gears than was fashionable 30 years ago, we use longer crank arms.
I just got back from a 18 mile ride or so; I had a meeting down at Brooklyn College, and it's such a nice day I rode back to the office instead of taking the subway. Average speed, through NYC traffic, around 15 mph. I'm a little hot and sweaty now, but knees are not sore at all!