The width of 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.

TIP: To more easily judge what size mirror would be suitable in a given situation, try cutting some newspaper roughly to size and pinning it to the wall in order that one has something physical to look at.

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 all the reflection above ones eye line will be of the ceiling, 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.