I installed a Tranquility with outside air in a cabin a few years ago. The air intake seemed large (and an odd size) for a stove with a small fire box. I used a 5 inch 90 degree round adjustable air duct elbow. I think I put the female end over the collar on the stove bottom. I remember it was a tight fit getting it underneath (so tight that it held it in place). Once in place, it opened to the rear and it was a straight shot from there out the wall. England's sells an outside air kit for 5 inch openings (AC-OAK5 $78.40) that utilizes a length of flexible metal duct which is another option. I went with the rigid pipe for durability. The outside air adapter mentioned earlier (AC-170AA) is the part that is already on the stove. I may head up to the cabin this weekend so I can confirm.
Thanks very much. I'd really appreciate a note on what you find. I wasn't sure whether to put a 4" inside the opening or a 5" outside.