So I plan to add a front disc to my offroad fixie and was wonder if I should go 160mm or 170mm rotor/brake setup. This would be my first foray into the world of disc brakes. Bike would be used for 50% offroad and 50% commute. I am 250lbs and would be using a nashbar rigid fork. Brake would be an avid bb7. Any advice would be much appreciated.