The key to the correct proportion is to size your mantel (or wall finish) first, and then work backwards.
A 36" or 42" fireplace opening width is more common, but you need to assume about 18" on each side for a typical non-combustible surround and wood mantel, 24" on each side for something more grand. The same thing goes for stone on each side. You need enough room to make it look like an actual masonry fireplace.
We tell clients to pick out their mantel first, and we'll fit the fireplace to it. And we always sketch it up in the room to see how it will look before asking the client to commit to anything .
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