What are the dimensions of a fireplace?

Fireplaces come in all sorts of sizes. There is no one standard size. Some are massive, big enough for a person to stand up in, others are very small, just big enough for one log.

Here are a couple of examples of some modern fireplaces:
A Bonaire electric 31-1/4 by 4 x 23-7/8 inches
A Dimplex electric 11-2/7 by 20 x 23-1/8 inches

Again, these are just a sampling.