This question comes up quite often and there is no one answer. However it can be answered by considering some of the following.
How big is the room? How high is the ceiling? Are there larger beams in the room? Is the stone present itself as large to the room? How tall is the fireplace structure? Does it go to the top of a vaulted ceiling or up two thirds of the way on an eight foot ceiling? What I think to be one of the more important items….what is it that YOU would like to enjoy in your natural piece of art?
Give me a call if you like and we can discuss. We are happy to provide you with any thickness. We are at times limited by our supply.
I personally think it is necessary that the size fit the room and that it would be off target if a visitor came into your room and remarked as to how gorgeous your fireplace is and then refers to your mantel as a “board”.