How to choose the right size overmantel mirror

 The uprights of the fire place should approximately match the sides of the mirror so that the perpendicular line of the fire place is matched by the uprights of the mirror frame.

Too wide, and it will overpower the base (fireplace) and too small, it will appear dwarfed by the dominant rectangle of the fireplace.

As to the height of the mirror, it depends on the effect that is required in the room. Remembering that the taller the mirror the more open and spacious the room will feel since any reflection above ones eye line will give the feeling of a more spacious room, whilst a shorter mirror will give more emphasis to the reflection of the walls and furnishings and thereby create a more cosy and intimate feeling.

TIP: To see the effect a mirror would make in a given situation, use cooking foil cut roughly to size required and pinning it to the wall in order that one has something physical to look at.