For any vanity, be it a 30-inch vanity or a 60-inch vanity, a mirror that is approximately the same width as the vanity is a popular choice.Matching your mirror width to your vanity’s width ensures a well-proportioned look. Of course, you can also select a slightly smaller mirror, if you prefer. For a 30-inch vanity, a mirror in the 24-inch to 28-inch wide range will create a more tailored or expertly “designed” appearance. Ideally, you’ll never want to use a mirror that’s larger than the width of your vanity as it’s likely to look visually off-balance.