It's generally recommended that the hood be 6" wider than the cooktop,
especially if it has no adjacent upper cabinets to help capture steam
and grease. CFM (Cubic Feet of air moved per Minute) also matters--the higher the BTUs, the higher the
recommended CFM. Anything above 400CFM requires make-up air, but codes
may vary, so check your local safety code. Also check the SONES rating--the higher the rating, the louder the hood.

GW discussions--hood CFMs

GW discussions--make up air

Start by putting in 8" diameter ducting if you have a range top. There is a lot of information already on the Appliances forum posted by member kaseki. There is no perfect answer most of the time.

General Information:

Width: 6" wider than cooking surface (so 42" wide hood for 36" wide rangetop)

Depth: If against the wall, 24". If in an island or peninsula, 27"

Fan Speed: I recommend at least 600 cfms, but check the Appliances Forum for more information.

Noise: Get an inline fan so the fan isn't so loud (as opposed to a fan that's outside the duct and directly over your head when cooking). Also, if you up-size the cfms, you can run it on a lower speed and reduce the noise. No hood will be silent, but with some planning, you can reduce the sound.

Appliance Forum Hood FAQ: http://ths.gardenweb.com/discussions/2766920/overview-of-vent-hoods

The hood's width at 6" wider than the cooktop is ideal, but many people have the hood the same width as the cooktop, and it's fine. Often people don't want to give up the 6" of upper cabinets, or cannot, because the cabinets are already there. If you have the space for a 42" wide hood, it would be the better choice, but is not an absolute requirement.

The thinking on hoods has moved somewhat from emphasizing the width of 6" wider, to more emphasis on the depth of the hood front-to-back. Many hoods sold these days are only 20" front-to-back, leaving the front burners uncovered. That is more of an issue than ensuring extra width on the hood. Find a hood that is close to 24" front-to-back if you can.

The height of the hood is ideally 30-36" between the top of the cooktop and the bottom of the hood. If you install higher than that, you are allowing too much space for smoke and grease to escape and float around the kitchen. Go lower than that and your cooking area will feel cramped.

I want to reiterate what Stan Z said about the correct ducting. You can have the best hood in the universe, but if your duct diameter is too small, your hood performance will be constrained. Most hoods require 8" or 10" diameter - the specs will say which. You can find hoods with specs for 6" diameter, but those hoods will be noisy, as air is pulled through a more narrow space.

600 cfms is usually enough for most people with a 36" width cooktop. Underpowering the hood is a mistake - you will have spent all that money on the hood, and it will be inadequate from the get-go. If you are thinking about going with more than 600 cfms, the advantage would be that you can run the hood on a lower speed setting most of the time, which will be quieter. The disadvantage of more than 600 cfms is that may require make-up air depending on your locality, building code, and how tight your house is.

Thank's everyone for the help. Didn't even think to go bigger than the range top. We won't have upper cabinets so not a problem there. Think I've narrowed it down a bit, now just on to the makeup air dilemma. Again thanks for the help.

When the time comes, we are going with the 36" Thermador range top as well and I've decided on a Zephyr 42" hood 1200CFM. We use lot of spices when cooking and lot of the food requires frying and not a pleasant aroma to people who aren't familiar with the food. I was told 1200CFM was too much but I wanted to have the option if needed. Another thing I was told is for the best effect we have to start running the exhaust 10 min prior to start cooking to get the air flow going. I do't know how accurate that info is tho.