Well, the author uses a 9 x 13 casserole, and that should be big enough tfor 6 "regular" eaters, if you're not serving anything else. By regular eaters, I mean no teen age boys, or football players. (You know, bottomless pits!
This sounds like a really versatile dish, one where you can use whatever you like in terms of bread, meat, and other flavors, so it needs never be the same twice (or ... the same company won't recognize it the next time you make it.) I LOVE this kind of dish!
I think I'd make two 9 x 13 casseroles full. Sounds like one wouldn't object to having leftovers for supper or breakfast the next morning.
Or, you could make one 9 x 13 and one 8 x 8. That would probably be exactly