The magnification of an object due to plane mirror is

If m is positive the image is virtual and erect .

  If the object is real, the image formed by a plane mirror is virtual, erect,of

  i) same size and at the same distance from the mirror.

ii) Magnification of plane mirror is $\text{m}=\frac{\text{size}\text{of}\text{the}\text{image}}{\text{size}\text{of}\text{the}\text{object}}$

(or)

In case of mirror, size of the image is equal to size of the object.