Well mine that I'm just finishing up is from an 80 gallon compressor tank and measures 24"x48". I built it with three racks that are removeable for some flexibility when cooking. I cooked three small (4.5 lbs each) pork butts this weekend and i could have fit 8 more that size on that one rack. I have no doubt I could easily feed 70 people with this size.
Personally I think that about a 24"x65" would be optimal. My door is about 32" wide, due to the cap weld seams, the extra length would definitely allow you to do a whole hog. I may be able to do a small one in mine, but I don't know.
I think that if you get much less than 20" in diameter, you get a little limited on space. The space under the RF plates, is going to be 5 inches or so, then a few inches from that to your first grate. So with a twenty inch tank, you're down to about 11 or 12 inches of headroom. If a pork butt is 5-6 inches tall, plus the thickness of a rack, you start to get pretty tight in there.