Intake port(s) size should equal or be slightly larger than exhaust port.
This one is quit a bit larger than yours
http://www.bbq-brethren.com/forum/showthread.php?t=162551
but a length of 4" exhaust tubing has proven to be plenty. I would think you will be OK with anything 2"-4". Go to you local muffler shop, can't beat the price of a foot or two of exhaust tubing. Match intake and exhaust size.
Ain't that for Horizontal offsets? ? ? I didn't see anything for internal firebox or basket or Vertical ? :twitch: