Disc brake rotors come in many different sizes, typically 160 millimeter, 185 mm, or 203 mm in diameter. However, many other sizes are available as brake manufacturers make discs specific to their callipers — the dimensions often vary by a few millimeters.
Larger rotors provide greater braking force for a given pad pressure, by virtue of a longer moment arm for the calliper to act on. Smaller rotors provide less stopping power but also less weight and better protection from knocks. Larger rotors dissipate heat more quickly and have a larger amount of mass to absorb heat, reducing brake fade or failure. Downhill racers typically run larger brakes to handle the greater braking loads and extended braking duration. Cross country racers typically run smaller rotors which can handle smaller braking loads yet offer a considerable weight savings of as much as 100g per rotor. It is also common to use a larger diameter rotor on the front wheel and a smaller rotor on the rear wheel since the front wheel does the most braking (up to 90% of the total).