Take each size for a ride & stick with the one that feels better. I would think the 17.5" would be a perfect fit, since I'm just short of 5'11 & 18" is my preference. You could also find a LBS that offers the Specialized BG FIT Service. I don't know if that would be practical for everyone though..there's only one in Georgia & I happen to live 10 minutes from it. Maybe where you live has more options though.