[nrm]

When we do an imu over here our pit is usually 2'x3' and we heat the rocks until they are white hot. Be careful if you are using rocks as they have water inside that will steam and make them explode. We did a 30lb pig, chickens, and a turducken for thanksgiving 2011 and used a wireless thermometer. Surprisingly the pig hit 180 in 2 hrs, and we pulled it at 6. Everything was cooked and tasted good, only thing to do differently would let the pig rest longer ~1hr and not do the turducken, it looked funny.
Forgot to mention how we prep the pig. Hawaiian salt the belly, 2-3 small rocks inside smells so good at this point. Lay chicken wire on the table, then stack banana leaves, ti leaves, pig, ti leaves, banana leaves and wrap the chicken wire tight. Banana stumps split on the hot rocks, banana leaves, food, ti leaves, banana leaves, soaking wet burlap bags, blue tarp, and dirt removed for the pit. Wait one hour and make sure no steam is escaping, if it is add more dirt.
Good luck and post pics of the finished product.