whether plane mirror always produces virtual image,erect and same size of the object in every position of the object

Yes , a plane mirror always produces virtual images . the rays of light follow the laws of reflection in which the angle of incidence = angle of reflection and the incident ray, reflected ray and the normal all lie in the same plane.

See the following diagram

The only point to be kept in mind about plane mirrors is that though the images formed are virtual and of the same size as the object, but the images are laterally inverted.