Wall Range Hoods:
Wall range hoods should as a minimum be the same width as the range top
Island Range Hoods:
Island range hoods should as a minimum be 6" wider than the range top. The reason is quite logical - island range hoods are exposed to cross drafts in the kitchen from ventilation systems and even people moving around.
Here is a picture of a 12' wide range hood - the range top is a 30" induction cook top and we provided a 600cfm internal vent blower with stainless steel mesh filters directly over the induction cook top - 30" wide. The remainder of the island range hood underside was custom fabricated stainless steel panels with lighting. The lighting is controlled by two dimmable range hood light controls - one controlled the kitchen hood lighting above the induction cooktop and the other rangehood light controlled the island rangehood lights on the perimeter: