This is a subject that deserves some research because you have many trade-offs. I can advise what the best thing is to do (functionally), but cost and aesthetics may force you to give up some performance. These are the basics for the hood:

The goal is capture and containment. Capture means getting the smoke into the hood; containment means not letting it escape except to the outside through the ducting. Hoods do not have the flow rate to suck up the smoke from the stove. They depend on pulling it into the hood when it reaches the hood. This means that to capture all of the effluent, the hood has to overlap it.

The smoke and grease effluent expands as it rises. The higher the hood, the larger the hood has to be to capture all of the expanded effluent. Commercial hoods are mounted seven feet above the floor and typically extend quite a bit beyond the cooking zone on all four sides. If wall and cabinets are present, they can act as "mirrors" to the expanding effluent and help duct it to the hood aperture. In such cases the size of the hood can be reduced somewhat.

For normal 30-inch or so hood heights over the cooktop, the hood needs to overlap the cooking zone (the array of pan bases, not the glass surface) by three to six inches on all sides. For 36-inch wide cooktops, the recommended hood width is 42 inches at a height of 30 inches or so. (Even this width might not capture everything if serious frying or grilling is done on an edge burner.) Hoods placed higher should grow to accommodate the effluent expansion.

The hood entrance aperture also needs to be deep enough (front to back) that it captures on those edges also. The hood center should be centered over the array of pan base areas. 20 inches is probably too small for a stand-alone hood. If the pan base zone is around 18 inches front-to-back, then one would want 24 inches at least for a hood 30 inches high, 27 inches if the hood is at 36 inches high.

A 20 inch deep hood would need "skirts" of some sort to direct the effluent up into the hood aperture. This would be impractical relative to buying a larger hood unless working with an insert within a larger cabinet exterior.

The CFM required is of the order of 90 cfm per square foot of hood aperture. This is based on a 3 ft/s effluent velocity from hot cooking. A lot of cooking is done at lower temperatures where this velocity is lower and the fan can be run proportionately lower and still contain the effluent. At lower speeds the fan will be quieter.

The fan should be sized for 1.5 times 90 cfm/sq. ft because its rated performance is measured when it is hanging in the air (zero pressure drop) but the reality of the duct losses, baffle losses, transition losses, imperfect MUA cause pressure loss and hence reduced fan performance.

I recommend spending a few weeks of spare time reading the various hood threads on this forum.

kas