Mountain bike disk brake rotors come in a variety of sizes, ranging from 140mm to over 200mm. Generally, the larger the rotor, the more stopping power it will provide. Large rotors also help dissipate heat better, which improves brake modulation and maintains power during long, steep descents.
The best brake rotors weigh less and provide advanced features for increased heat dissipation. Some rotors aren't just one piece of metal but are rather two pieces, which allows the rotor to float between the brake pads, lining up perfectly. Shimano also sells IceTech rotors, which feature two different types of metal for advanced heat reduction.