Given,

Magnification by plane mirror= 1

We know,

Substituting value of magnification in this formula:

Height of object = Height of image

So, the given information signifies that image formed by a plane mirror is of same size as that of object.

Also,

Substituting value of magnification in this formula:

Object distance = - Image distance

Or image distance = - Object distance

So, the given information also tells that image formed by a plane mirror is at same distance behind the mirror as the object is in front of the mirror i.e. virtual and erect image.