When you say riding all mountain, it usually means that your going to be cruising around the mountain. In that case, a bigger board is going to be more stable when you get going faster. However, the only reason that people do not ride big boards in the park is for easier maneuverability. So yes, it is true that people will ride bigger boards if they ride all mountain, than they would in the park. However, do not get to big of a board as that might make it a little difficult to control.