What length for nostril piercing

My nostril piercing (6 months old) isn't reacting well to the sterling silver jewelry i got from my piercer. I'm going to buy a neometal titanium flatback labret from body art forms but I have no clue what length to get. I know my piercer used 18 gauge jewelry but I have no clue what length to get to ensure it doesn't move around and cause irritation, any tips?