The side sleeper generally needs a thick to firm pillow. The space between the neck and shoulder is rather large, so it is necessary that this space is folded as well as possible. You should also consider the firmness of the mattress. If you have a soft mattress, you take the rest of your life away as you wish. If you now have a firm mattress, you need a firmer pillow.
The pillow ensures maximum relaxation of the spine. This is achieved when the spine is fully supported and all vertebrae lie in a smooth line. The choice for a type of pillow starts at the door your post sleeping position. Three types of sleepers can be distinguished: the side sleeper, the back sleeper and the abdominal sleeper.