I’m installing a range hood with 6″ ducting.  Because of the placement of the range, joists, etc., I need to sneak past a radiator pipe to get through the wall and into the porch roof, by stepping down to 5″ for about 1′.  In the porch roof, the duct runs for another 1′, turns 90, and runs another 1-2′ to get to the wall cap.  My question is: should I step back up to 6″ on the other side of the wall for the final 2-3′, or leave it at 5″?