I used one before but don't know the demensions. There are several versions by the way. There are some you put coals on top, sides and bottom. These cook the fastest but you can't check them until you take the thing apart.
If I could weld, I would make one about 3' by 2' in width and about 10" deep out of thick aluminum. That way you could fit a decent split back pig in it. I would also make about a 2" lip on the lid with a handle(hook) in the center so that it could be picked up with the coals on it.
The way we used it was digging a trench coming out on both sides about a foot. We would start coals and the begin piling them on top all the while keeping coals going on the bottom. It's an all day affair but the outcome is as good as it gets.
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That's why they call it fishing and not catching!