Rotors come as small as 140 millimeters in diameter for road and cyclocross applications, all the way up to 205mm for downhill (DH) mountain biking. Generally, road and cyclocross use 140 to 160mm, cross-country (XC) mountain biking use 160mm, trail riding use 160 to 180mm (sometimes a mix, with the larger rotor up front), enduro uses 180mm, and DH uses 200 to 205mm. Larger rotors are able to dissipate heat over a larger surface area but are heavier, so you’ll want the smallest rotor you can get away with for the type of riding you generally do.