You need at least 5" to get to the skeletal muscle. 6" if you want to bulk up your bones!
in seriousness though... it really depends on how much fat you have on your ass... if you have 2" of fat, then 1" isn't enough, as it has to be in the muscle. Then again, if you're totally lean and only have 2" of muscle on your ass, you may not want to go with a 1.5" needle. 25G should be good though. Change the needle tip between drawing and injecting as it blunts the tip slightly.