You should not feed more than 2 lbs of rice bran a day. It is 40% fat and a horse cannot metabolize more than 2 lbs of rice bran effectively. Make sure the rice bran you get is "stabilized" as well. It can go rancid. Stabilized rice bran has added preservatives to make sure it doesn't.
Rice bran is just a fat supplement, to be used with your feed. It has almost no nutritional value aside from the fat content.