Yes, that hood looks undersized for the range, though that might be illusory. There are a few very thin hoods out there. I don't know if independent liners come that thin. I just saw one that was just a couple of inches, though I don't know at all how well it worked. My own new hood (not flat at all so no comparison) actually seems to make more visible steam in my cooking because it pulls it up so much. The picture might be deceiving, however. It looks at least 6-8 inches tall to me. The main thing about flat is that the route for the air from the sides isn't as direct so you need a good blower.

Most recommendations: Width...6" wider than the cooking surface (so for 40", 46" would be the recommended width). This allows 3" on either side to increase the chances of the hood "capturing" the grease/smoke/steam/odors that spread out as they rise. Depth...At least 24" deep. If your range sticks out much past 25.5" from the wall, you might consider the 27" depth. CFMs...it depends on (1) what/how you cook, (2) the size of your hood relative to the cooking surface, (3) how high you mount it, and (4) the number of turns & length of duct run you have. Higher CFMs......If you undersize your hood (o turn...get higher cfms (1200 is probably fine for the vast majority of people) Recommended CFMs......If you size the hood appropriately, don't do a lot of frying, mount it according to specs (usually no more than 30" off the cooking surface), have a short duct run, and no more than one 90o turn...you'll be OK with the recommended. I'd say 600 to 800. Our hood is 42" wide over a 36" cooktop. It's also 24" deep. It's mounted 30" off the cooking surface. We're a relatively tall family...ranging from 5'10" (me) to 6'6" and still growing (my DS). Only once has my DH hit his head on the hood....and that was when he stuck his head up into the hood to see it working. When he took his head out, he didn't duck low enough and hit the back of his head on the hood! None of the rest of us has hit our heads...at least as far as I know. [I did ask once and both my DS & DD denied ever hitting their heads. My DH has not hit his head using the hood properly :-)] I think what happens is that your body and mind are "aware" of the hood being there (subconsciously or consciously) so you don't hit it.

The easiest way for them to run duct in a two-story house is between the floor joists in the ceiling above. Well, the easiest way is probably to run through upper cabinets, if you have them. You need to figure out which way those ceiling joists run - look in an upstairs register or at the joists in the basement or attic to get a good guess. If they go the right way your ducting could go up from the hood to the ceiling, turn a right angle to go between the joists, and go out through the wall. We had a guy from a fireplace shop come and look at our house. He had good suggestions for how to get to the outside from an interior wall, and it wasn't what we thought it was going to be, mostly because the joists ran in an inconvenient direction. We were ballpark-quoted in the $1000 neighborhood to go completely across the top of the kitchen and out the opposite brick wall, plus a second run of duct in another place, through another brick wall, for make-up air (required for a 401+ CFM hood in our area). I believe if we only had to do the single run, it'd have been about half that. Material costs are very low and the hard work in our case was getting through the masonry. Note that our estimate did not include patching up the ceiling after they cut a bunch of holes in it. We also have a recirculating microwave-hood and we hate it. The diffuser piece at the top on ours has broken so I get the breeze right in my forehead as I stand at the stove. We can't wait to have a proper outside-venting hood!

You need to completely cover the cooking surface...so use that as a guideline. If possible, you also want it to be 3" in front of and 3" behind the cooking surface...So, if the cooking surface is 18" deep, you want the hood to start out 3" before the cooking surface & extend 3" past the back of the cooking surface. Ditto on the sides...that's why it's recommended your hood be 6" wider than your range/cooktop...especially on an island or peninsula! Don't worry too much about hitting your head...our 24" deep hood is over our cooktop on a wall so it's only 1.5" in from the edge of the counter. We don't hit our heads on it. Our hood is mounted 30" above the cooking surface and we range in height from 5'10" to 6'6". The only time I remember anyone hitting their head is my DH...and that was when he was peering up into the underside of the hood...he hit his head on the way out not on the way in! My entire family uses the cooktop & hood from time to time and we've had no problems!