If it is a flat mirror, there is no difference in image size, but of course this only holds if parallax is ignored. In the case of poor eyesight, due to the influence of round and square frames, we feel different in imaging.

Optically, only the concave and convex of the mirror will affect the fat and thin image.

The convex mirror makes the image smaller.

Concave mirror makes the image bigger.

A flat mirror does not change the size of the image.