One of those components is the fireplace hearth extension. For those of you who don't know what that is, it is that hard surface area in front of the fireplace in the picture to the left.
The purpose of the hearth extension is to protect your floor and your home from fire. But how big (or small) should the hearth extension be?
It all depends on the fireplace. For the enclosed fireplace in the picture above, the hearth is actually just decorative. Because the fire box in sealed from the interior, it is not really needed.
If you have a wood burning fireplace (or even a gas one in an open fireplace), a wood stove, or other type of unit, the hearth extension should be about half the width of the widest part of the fire box. So if the widest part of yours is 40 inched, the hearth should extend at least 20 inches from the opening of the fire box.