The bigger the rotor, generally the more stopping power you can have. Also, bigger rotors do a better job of resisting over heating. If you ride a lot of DH, you wan bigger rotors. If you ride a lot of XC you want smaller rotors.
The coefficient of friction of some brake pads can double from when they are cold to when they are warm which means you get twice as much stopping power. If you use rotors that are too large for the intended purpose the braking system can actually run too cool resulting in weak braking. You need to select components so they are used in their optimun operating range.
Obviously weight of rider will have a large impact on how powerful the brakes nee to be too.
You need to make sure that your fork was designed to be used with larger rotors before putting larger rotors on it. Some forks cannot handle the additional stresses imposed by larger rotors.