Water and milk cannot use the same size of teats else babies will tends to get 'choke' as water is 'smooth' while milk is thicker hence, cannot use same size.
You can actually self test the teats yrself with the water and milk (which I done so to convince myself that time). You will notice that the water will flow out quickly while the milk is just dripping out.
What I did (until now) is: Milk - M size teats, Water - S size teats.
If you know, there's also 'Y' size of teats, that is either for teats beyong M size or, to feed juices.. or some mummies add cereal into the milk (which is not recommended by pediatrician) will use 'Y' teats.