If no one else is going to field this one, I'll be glad to.

Since you're quoting residential code for masonry woodburning fireplaces, I'll assume that's what you have. Manufactured fireplaces (gas or wood) are governed by the installation manual from the manufacturer.

What would look best? Good question, with a simple but not very helpful answer: <b>It depends.</b>

When working with stone, one factor is how the stone will "weave" as you go up the sides of the fireplace opening. When installing stacked stone like in the phot above, you want to be care not to have joints line up vertically too often or have such a small area to work with that you can only fit one stone between the mantel and fireplace. For that reason, I don't recommend any less than 12" wide on the sides, even though you aren't required to have that much by the code.

From there, you can match the width on the top between the fireplace and the mantel header, or increase it slightly.

The proportions are good on the mantel above, but the whole arrangement is pretty massive. Assuming it is a 36" side fireplace, there is probably about 12" of stone on each side, and the legs of the mantel add another nine or ten inches, so the whole thing, by the time you add the projection for the crown moulding and mantel shelf, is probably about 7'-8' wide. Pretty big.

Doing the same math for the height, with about 29" opening height, 12" of stone and probably about 18" of mantel header, crown and shelf, your total is about 59", just short of five feet tall.

I'd suggest trying out the proportions with masking tape on the wall to see how it fits, or sketch up the whole thing (like below) so you can see how the proportions work in your space.