Why does plane mirror form image of same size as object? 
Plane mirror form images of the same size as of the object.

Also if we need to see ourselves completely in mirror, we would require a mirror of at least half out height.
Assume I am 6 feet tall then if I use a mirror 3 feet tall then how come me and my image have that same size, should not my Image by 3 feet tall and if yes then why we say that the Plane mirror form images of the same size as of the object?
 A: The optical ray diagram of a plane mirror may help

Also here:


Let’s say you have a toy car, and it’s sitting in front of a regular bathroom mirror. The distance between the car and mirror is called the object distance, and it’s always positive. If you look at the image of the toy car in the mirror, it will appear to be the same distance behind the mirror as the real car is in front of the mirror, at the same height. It will also appear to be the same size as the real car. The image of the car looks like it’s behind the mirror (and the light we see does not directly emerge from the image), we say that the image is upright and virtual, and that the image distance is negative.

Because of the geometry of optical rays, plotting them, and measuring the sizes , plane mirror images have the same size as the original.
A: Given great enough distance, I see a tiny mirror eventually acting like a pin-hole camera.
While in elementary school, I had a broken piece of mirror (maybe 3-in by 1-in) and it reflected the light from the sun in the shape of the broken piece onto a nearby wall.
However, the further I projected the reflection the more circular it became and the larger the image (so even in this distance it was acting as a pin-hole reflection)! (This also works for a solar eclipse, cast the sun's reflection on a house (which is of course shaded) maybe 200-ft away, go and examine the image (inverted as through polar inversion), and you'll see a large and good resolution image of the eclipse.)
Therefore, if you reflect the image of the sun back to the sun, even with the broken piece, the light arriving at where the sun was before it had moved (due to the Earth's rotation and the time it takes to arrive to us) by travelling the same distance would be the same size as the sun.
