If you look it up at wiki, it sounds like it refers to the whole oven. I found dictionary definitions including:
- A brick, stone, or concrete area in front of a fireplace
- The floor of a fireplace
- The floor of a fireplace, usually extending into a room
I guess it can mean just about whatever you want? Personally, I think "floor" is the most common and least confusing way to describe the cooking floor of the oven.
When I asked, I asked because I wasn't sure if you were asking about the diameter of cooking floor or width of the door.