The 'breast' is the whole fireplace that you see in ya room.
The 'flue' is the narrow passage to the top takes the smoke(this isnt straight it is bent to draw the smoke up
The 'Stack' is the bit on the roof.

You can remove all the loose bricks. You should end up with a hole as deep as the actuall breast....ie the far back of the hole(fire place) should go as far back as the actuall wall in the room(the wall on your pic where the socket is)

It will be as high as where it starts getting smaller on ya pic and as wide as it is at the front...

Dya get me?