Hard to believe no one has jumped in to answer this, so I will...

The "rule" is that the vent should be wider than the cooktop by 3" on each side. So a 36" vent for a 30" inch cooktop.

That said, I don't think you always need to follow the rules. It really depends a lot on the type of range you have (gas/electric) and the type of cooking you do. If you have an electric range and don't so alot of frying or high heat cooking that generates smoke, you would proably be fine with a 30" vent over a 30" cooktop. It's still gong to be better than downdraft...

Interesting.

I am doing a 30" open burner gas, and have been told to go to a 36" hood. I also thought of getting a customer stainless steel panel to go between the range and the hood - but what size - 30" would fit nicely between the 4" high granite back splash, but wouldn't that look weird when it meets the hood.

The purpose of a hood is capture and containment of cooking effluent: smoke, odor, grease, and gas combustion products (with a gas range). At a minimum, each gas burner, electric coil, or induction hob heats a container and causes evaporation of some of its contents. This effluent expands as it rises.

The ideal case is to capture and remove all of this expanding effluent. This requires the hood overlap the cooking containers by the amount of the expansion. Otherwise, the uncaptured effluent has to be removed from the home over time by air replacement. Grease settles, so it doesn't remove so easily.

kas