I am building a house and need to choose a firebox and mantle. I'm between 42" and 50" opening for the firebox (around 26"-30" high). My living room will be 21'x25' with 12' ceiling. Is there a rule of thumb for fireplace size and also mantle size? The wall that the fireplace will go on is 90" wide.... should the mantle take up the entire 90"? How wide should the granite surround be? How wide should the mantle legs be? Any advice on proper scale and proportion of fireplaces, surrounds, and mantles will be greatly appreciated. Thanks in advance!