Published at Monday, July 16th 2018, 09:52:07 AM by Iva Powell. Bathroom Mirrors. The size of your mirror depends on the size of your vanity. Your bathroom mirror should not be bigger than your vanity, because if you are using sidelights in your bathroom, then the exterior limit of the lights can go beyond the vanity. Mirror lights can be a good choice for a small bathroom, but in a larger bathroom, one extra ceiling fixture is essential for general lighting purposes.