I'm a newbie here. I have a fireplace that I need to measure to see which size I need to get -- large or small. I'm not sure how to properly measure it. The fireplace is an outward curve with a concrete mantle. The firebox comes out 15 inches with brick at an angle. Then there is a cement mantle topped with an asbestos metal plate. How far should I measure from the back wall of the fireplace to the front?