Answer:
The size of the mirror must be half of the size of your face.
Step-by-step explanation:
As we know that pocket mirror is a plane mirror, for the plane mirror, distance of a person from the mirror does not affect the image formed by mirror. But the height of the mirror must be half of the height of the person to see the whole face. It is because pocket mirror is between your head and the virtual image formed, to see the virtual image, your face rays must strike the reflecting surface, for that purpose the size of the mirror must be large enough so that your face rays can strike the reflecting surface easily.