answer is h/3
let the distance between the mirror and the observer be X
the height above the center of mirror of the wall be = h/2
the mirror is symmetrically placed on the wall - symmetric about the center of the wall.
The wall is at 2 X distance from the mirror. So the image is formed at 2 X distance behind the mirror.
The size of the image will be the same as that of the object. The size of the image above the center of mirror will be h/2.
Now, (h/2) / X = tangent of the angle of the light rays with the axis, the rays reflecting from the top of the mirror.
Also, (h/2 ) / (3X) = (height of mirror) / X
mirror height = h /3 --- one third of size of the wall.