I decided to look up framing a mirror and found this SITE. It might be just what you need. Check out their "See How Easy" video, then click on Shop to see their frame choices. You can also order $1 samples to view in your home. I don't think there is a rule for frame size, but most of these seem to be about 3 inches wide.

However, there are guidelines for vanity lighting and [HERE[(https://www.houzz.com/magazine/how-to-get-your-vanity-lighting-right-stsetivw-vs~57853042) is a Houzz article on the subject. Sconces set at eye level on either side of each sink are considered to give the best lighting, but you would have to drill through your mirror to do that. Instead you should put a multi bulb fixture above the mirror over each sink, or three fixtures that cast light from each side of each sink. The author, Janell Beals, has many examples of this in the second half of her article.