How can you tell if your breast pump teats and your nipple sIze match each other or You need to get a smaller/ bigger size?

I think Izda;s explanation is pretty thorough. Trial and error is the best method unfortunately. I was using teats that were too small and it made my nipple really sore as it rubbed against the sides and was super painful. Pumping should not be a very painful exercize so if your nipples feel like theyre being pulled or squeezed, or are getting irritated getting rubbed against the walls, chances are your teat size is wrong.