I have a Tri-Cross and am 5'5". It is the 51. I could not get on the larger bike as it was just too tall. If you can possibly try it for size, I would definitely do that. Even if the price is right it might not work for you! Good luck. It is a great bike. I do not cyclocross. I use it for touring! I also have a road bike but the TC is really comfortable for long rides.