Generally, people who are used to lying on their backs should have the same height as their fists. For people who are used to sleeping on their side, the pillow height should be consistent with the shoulder width.
Ancient Chinese medical books once pointed out: "Lying on your side is just shoulder height, then lying on your back is comfortable." In other words, the height of the pillow should be level with the trunk when lying on your back, that is, the pillow is one punch high when lying on your back and one punch and a half high when lying on your side. Generally speaking, the height of the pillow is 10 ~ 15 cm, and the specific size generally depends on the physiological radian of each person.
Of course, when sleeping, you can also put the middle of the pillow flat and raise the pillow to the neck according to your personal habits, as long as the cervical vertebra is not forward or sideways, and you can maintain an appropriate backward extension posture. So you can avoid cervical pain when you get up in the morning.