The standard mirror size for a 48 inch vanity is between 42-44 inches. This measurement allows you to mount the mirror above the vanity, leaving a gap of 2-4 inches between the mirror edges and the vanitys corners. This gap ensures that the mirror looks proportionate to the vanity and leaves enough space for the plumbing fixtures. It is recommended that the mirror should be a few inches smaller than the actual vanity. If you want to add a light, the mirror size for a 54-inch bathroom vanity should be between 48 to 54 inches wide and 24 to 30 inches high. If you prefer a round mirror, for a vanity that is 48" wide, youd probably want to get a mirror thats between 20" to 30" inches in diameter.