Originally Posted by willtsmith_nwi
I think people get sold on the "POWER" of 8 inch rotors. Very true, they do express more leverage on the disc at a greater distance from the hub. But we have to remember that there is another "force" at work here. It's the force of friction between the tire and the ground. And if the power of the brakes exceeds your traction, you'll lock up the wheel.
The only "power" you need, is enough to lock up the wheel under a given condition. 8-inch rotors primary benefit is a greater surface area with which to radiate heat. In cross-country, you'll never get your 6-inch rotors that hot. In downhill, you're likely to warp a 6 inch rotor.
Most V-brakes have the capability to lock the wheel also - but that's not the only factor. It may be worthwhile to note that due to the increased leverage, larger rotors require less effort at the lever, requiring less input and resulting in reduced hand fatigue.