Aperture size from firebox to smoking chamber:
On my pit (20x52" smoking chamber), the aperture is 15" wide by 5.5" tall, like a semi-circular piece of pie completely cut from the bottom side of the smoking chamber. There is a semi-circular baffle (essentially the part that was cut out), angled from the top edge and protruding into the smoking chamber to deflect heat and smoke initially to the bottom of the pit. See this picture:
You can also see an example of an angled baffle from the smoking chamber side in Simon's second picture in this thread:
As far as the location of the tuning plates is concerned, I don't have them, but on a 24" pit I would think they ought to be further away from the cooking grate than 3 inches.
See this thread:
And this one where PitBull experimented to find a way around his radiant heat problem coming from the plates:
As far as the question la coonass
had about firebox size, 20x20 square or 24x24 are pretty common sizes. I have a 20"Wide x 24"Long square firebox on my 20x52" Klose which also has an upright on the end. That size firebox is more than sufficient for the pit. It had plenty of room around the fire.
See this thread for examples showing my fire size: