Start with the shoulder gap
A side-sleeper pillow needs enough height to fill the space between the mattress and the side of your head. Too little height lets the neck bend down; too much pushes it upwards.
Mattress softness matters too. Your shoulder sinks further into a soft mattress, reducing the pillow height you need.
Look for stable support
A supportive pillow should settle under pressure without collapsing through the night. Contoured memory foam is one way to combine a softer head cradle with firmer neck and shoulder zones.