It's probably too late, but for information (and if you haven't already done so), I suggest posting questions like this on the "Discussions" side of Kitchens. This is the "Gallery" side that's more for finished kitchens and the like. The Discussions side of Kitchens is for on-topic discussions concerning kitchen remodels. You will most likely get many more responses over there. Good luck!

Concerning more ventilation: Most of the time it's fine. I don't mind cooking smells in the house. I've never cooked fish or something with strong unpleasant odors. The only times I really want more is when I'm sauteeing onions or searing steaks and I get smoke going. I turn it on in those cases but it doesn't even have enough pull to really pull the smoke up to the vent if that makes sense. Also I have greasy build-up on top of my cabinets. I had no idea that a fan was even suppose to be used for greasy things (can you tell my mom never used hers either), so now that I know a fan could help with that it would be nice if I had one that worked better so I didn't have to scrub the top of my cabinets every year. As far as current venting goes; if I had any it would have to be in the existing wall, nothing goes through the cabinet over the stove. Also there is nothing on the roof or side of the house that indicts anything being vented anywhere in this area of the house. Weedmeister, after reading your message I realize I was incorrect in my thinking of where the duct connects to the hood. I was thinking they would connect to the back side of the hood (where it touches the wall) but after reading this I think you are saying the duct will connect above the hood. Is that correct? In that case I'd have 2 turns in either direction (one from the fan to the wall and one from the wall to go up or to the side. I'd definitely want the vent in the wall because I have a huge open space above the cabinets up to the cathedral ceiling and I wouldn't want to see a vent running there. Thanks for helping me clarify this stuff. More and more it sounds like going ductless might still be my best choice. The filters on them were rather expensive and I was starting to think maybe over the long term that just installing the ducting work would be cheaper than replacing all those filters. So if I'm going to install a ductless hood, does the CFM's matter so much. I was considering the Broan Evolutions QP130 or Broan QP230 hoods.

As a rule of thumb, a 6" diameter exhaust duct would be appropriate up to 600 cfm. If you're installing a 48" range or cooktop you'll need a higher air flow rate than that. The G.C. and the mechanical contractor installing the hood should have run into a similar condition and know a couple of options for a work-around. If they don't, they either just started in business or the scope of their experience is pretty narrow. If your G.C. or mechanical contractor needs to be "educated" by houzzers, they're in trouble. And so are you.

@3onthetree is correct, within the assumptions presented. However, so long as you are satisfied with well under 200 CFM, you can transition to 4-inch duct. (With the specified 6-inch duct, the blower would probably only push 300 CFM at best, and this assumes very low pressure loss in whatever method you have of providing make-up air. There will be slightly less overall duct pressure loss if the duct to the ceiling is 6 inches rather than 4 inches, but this difference may be trivial depending on how long the 4-inch duct is to the outside. Impingement grease buildup from too high an air velocity may be compensated by not likely actually achieving a very high velocity. Note that even 200 CFM in a 4-inch duct results in a duct velocity of 2300 ft/min, undesirably high, and quite possibly beyond the capability of the hood blower. Ventilating to the outside will usually beat recirculating, but we don't know enough facts to be sure in this case.