Well John, I beg to differ. At 17, you're almost certain to grow a couple more inches. In my experience, when in doubt, go larger. At 6', most of my bikes are 58cm. One is 57, and one is 61. I don't bother with 56, as it's just too small. Look at the top tube measurement as well, when shopping for a bike. That 61 I have is a hair long in the top tube (59cm), and I adjusted with a shorter stem. 59 cm might be fine for you. Keep your options open. Ride both sizes enough to be as sure as you can be.