This is going to sound SA but...heavy enough to kill a rabbit.  If you have a good throwing arm, it can be lighter, if you are average, it needs to be heavier.  Personally, I like over a quarter of a pound and less than half a pound.  As far as length, that depends on where you are throwing, eastern woodlands will need a shorter one than desert plains.  If the bend is not correct, the rabbit stick will spin on its long axis instead of rotating.  Bill

  Hopi rabbit sticks were an airfoil design and were very light, they were meant to hover just above the ground and strike a rabbits legs, breaking them, not killing the rabbit.  Other rabbit sticks are heavier and are meant to stun or kill and others don't even have an airfoil design and simply are a curved stick or are sharpened on either end.