On the design/theoretical side, larger rotors will exert more reactive-torque for any given clamping-pressure from the pads (generates more deceleration). In the practical usage end, you run into manufacturing, cost, weight and maintenance questions as well. So there's a balance of sorts. The leverage-ratio of the lever-to-caliper typically incorporates a certain rotor size in mind. And the curvature of the pads are designed for a certain rotor size as well. In the end, as long as the components match and fit, that's all you'll want to be concerned with.