It is particularly important to get the right size mirror when it is being placed over a fireplace since the hearth tends to be the focal point of any room. The width of the frame should therefore approx follow the width of the uprights supporting the mantelshelf, and not the shelf itself. The height is dependant on the effect which is required. Remember that any mirror glass above the viewers eyeline will reflect thr ceiling which will make the room feel more spacious and airy.Conversly, shorter and wider mirrors such as the New York Rosebud will reflect the furnishings of the room and make it feel more cosy and intimate.
A useful tip is to pin or stick aluminium cooking foil onto the wall where the proposed mirror will be – this helps to get the right proportions as well as showing you what the finished mirror will reflect. If a mirror is to be hung over a piece of furniture, it usually looks better to have the width slightly smaller than the width of the base that it is visually sitting on