You need to check code. When we re-did our fireplace and put gas logs in, we needed to have 12" of non-combustible material on either side of the firebox before we hit the wall/surround. Check to see if that's the current code.

For our new build, I talked with the manager of a fireplace store. He told me to bring my plans in with the dimensions of the room to help determine the right size of the fire box and thus the size of the fireplaces for the spaces. They know what "feels" right and what would look too small in the space.