When choosing a new bathroom mirror, there area a few details you need before you shop.  Begin by measuring the wall space.  A good rule to follow is that a bathroom vanity mirror should be no wider than your bathroom vanity, and ideally it would be a few inches smaller.  The height can be as tall you want, but you need to take any lighting fixtures into consideration.  Moving light fixtures can be costly.  As far as finish, I recommend you go with a different color than the finish of your vanity, as trying to match finish colors across manufacturers is very difficult.  If you go with something different, it will look more natural.