Disc brake system for downhill and dirt trail I have a 2000 GT XCR4000 I would like to put disc brakes on. What size of disc brakes would I need to do this upgrade the brake system for downhill and dirt trail ride.
Probably 200mm for down hill. 180mm will work if you are not too heavy and don't ride hard. 

Disadvantages of bigger rotor is heavier and more prone warping and getting damaged. Advantages are more stopping power and better heat dissipation. However, that's presuming same quality brakes. Better brakes trump a bigger disc.

Does you bike have mounts for discs?